No-one's posted one yet, so I thought I'd give it a go.

The Candy challenge isn't just a puzzle, it's 7 puzzles that can all be worked on at the same time. That can lead to a lot of confusion in trying to explain it.
So, I'm just taking one puzzle at first to outline the basic concept. I'm also assuming a starting position with 3 different shapes. I realise this not necessarily what reality has given you, but it makes it easier to explain.
The Candydevil puzzle.
The end product is being able to attract the violet Candydevil. For that, all the candies have to be a certain shape, a certain colour and in a certain order in your garden.
The challenge is to find out what shapes and colours they are. To do that you first need to try out the shapes.

Prepare yourself witha chart, there are a few that players have created for you to copy and paste, or make your own. This is the one I made (others may wish to post their links too)
https://docs.google.com/spreadsheets/d/1lfdpJB0LkxWdhrlLRjCJmygi8L_s6F6X-rSl1o9hQuM/edit#gid=0

STAGE 1: 1
With 3 ready to harvest candies in your garden, put out plates of wild berries.
If a red candydevil turns up, the shape on the left is the correct shape, but the wrong colour
If an orange candydevil turns up, the shape on the left is the correct shape, and the correct colour

If a yellow candydevil turns up, the shape in the middle is the correct shape, but the wrong colour
If a green candydevil turns up, the shape in the middle is the correct shape, and the correct colour

If a blue candydevil turns up, the shape on the right is the correct shape, but the wrong colour
If an indigo candydevil turns up, the shape on the right is the correct shape, and the correct colour

A - There is a possibility that your set up is attracting 6 different things (At this point, remember that wild berries also attract Cherrymoles), so give it a good try before moving on.
Note the results you have got so far for correct shapes and for the colours you have tried, note if any were correct (at this stage it's the shapes that are important)
B - then move the candies into a different order (I'd move the first to the end) and repeat above
C - then repeat that move from the same end and repeat again.
So your candies have been in the orders (eg) ABC, BCA, CAB.

STAGE 1: 2
Next you will need to try 3 different shapes and repeat all the above with them (That's assuming you haven't found all the right shapes yet).
There are 7 shapes, but if you have tested 6, the 7th will be wherever you still have gaps (in an ideal world)

STAGE 2
After trying out the shapes, you should know all the shapes you need to get the violet Candydevil (also the violet Cherrymole).
The next stage is to find the right colours.
Hunt for the shapes you need and make sure they are in the right positions (left, middle, right).
Any in colours you have already tried or as soon as a red, yellow or blue Candydevil comes along, you know you can harvest that candy (but don't leave yourself short of the 3 candies).  Keep the correct ones and hunt again for the remaining shapes.
Always note the colours you've tried whether correct or not.
Once all 3 shapes and colours are correct, you will be able to attract the violet Candydevil. It may take a while to come as you will also be attracting orange, green and indigo Candydevils (and possibly some Cherrymoles)

REALITY CHECK: It was easier to explain by assuming 3 different shapes to start off with, I realise that's not always what happens in real life.
I also realise that if you get a right colour in the first 3, you might not want to harvest it.
The principle of trying out the shapes first remains the same.

That's the basic puzzle, so now to the whole thing.

I mentioned that you would also be getting the information on Cherrymoles while doing Candydevils.
In the same way - by using different foods, you can also be gathering the same information for all the other Candy critters.
Trying to solve 7 different puzzles at the same time is where it can get confusing.
At first every shape is new and there's something to be learned from every candy in our gardens (even if they attract nothing in any position, on any food type - that shape can be eliminated ) the first bits of information will be all over the place, but the gaps will start to fill up and hopefully it'll start to seem more orderly.

So at each of the Stages (and sub-stages) mentioned above, you would need to take 4 times as long as you do each with a different type of food. (Even if you have different foods out at the same time, you would need to take longer to give each type a good try-out)

Now you know which 3 shapes to use for each type of candy critter. (you might even know a few colours).
Next stage is to find the colours.

You could of course do this as soon as you know all the shapes for the violet of any candy critter, or wait until you know all of them before going for the violets.
I'd do one section at a time (or using a greenhouse- do a pair with the same food type), keep hunting for the 3 shapes for the type.

Each time a red of the type turns up, the colour of the candy on the left can be eliminated. If an orange turns up, the colour is right so keep that one.
Same with yellow (wrong colour) or green (correct colour) for the candy in the middle
Also with blue (wrong) and indigo (correct) for the candy on the right.
Keep replacing the ones with the wrong colour eliminating colours as you go until they are all correct.

Note that if you have filled gaps with an untried shape, it's worth checking that all the gaps you filled with it are actually correct  when you first need to use it.

this is cool Ruth
that roughly what I am trying to do, I created an excel file that I am filling with the wrong and right shapes and colours, but at the end it will need time all this :) good luck to all and have fun!!!!!!!!

Thank you!  It gives me hope that some day in the distant future I will actually be able to attract violets!

This is what I'm doing, Ruth, with a slight twist --- I'm inserting symbols that match our plant shapes in my spreadsheet for the shapes, so at a glance I can see what shapes are needed and what have been eliminated for each critter.  Color coded, of course!  It made it easier for me to track!

Thank you Ruth. After reading this over a couple of times and checking out how you set up your document I think I have a better understanding of how to go about this :)

THANK YOU RUTH ... ... I've been lying awake in bed at night thinking over all the instructions and trying to visualise a chart of some kind. This is brilliant. Most of the conclusions I've drawn are exactly that - drawings. Not words.

So my question is about the colours of the SHAPES, of which at second glance there appear to be eight - colours of shape.

I know that the shape colour system is completely separate from the candy crittur colour system. My question is about the names for each shade of shape. When in Fairyland, blues indigos and purples are often, shall we say, surprising!! For example, Buttapillafloras, where blue is turquoise, and almost indistinguishable from green which is a very bluey green, and indigo is blue. Another example is the Summer Fruit Rainbow, where blue is indeed a lovely blueberry, but indigo is a very purple aubergine, and violet is a pretty but pink jackfruit!! It's nice to have variety, but I just want to be sure on the naming thing before I devise my spreadsheet, not discover a new surprise afterwards.

How many are there? and which purple blue and pink is allocated to which colour name?

Red = RED
Orange = ORANGE
Yellow = YELLOW
Green = GREEN

So far so obvious.

Then there's a pale blue which = BLUE I presume

because...

cobalt/ornery blue = INDIGO I've either seen a label somewhere for this, or inferred it from a discussion about a garden

so...

the purple one (which in some other fairyland rainbows might be called indigo) = VIOLET?

and the pink one = WHAT?! because if this was a jackfruit it would be standing in for purple.

Not obsessed about colour names. Not at all. No way. Uh-uh.

But yes, I am needing to know! and no, I can't do Candy Critturs right now, I'm desperately leveling up on, er, something else.

Thank you. I made my own charts according to my thinking - but they are much the same principle. Still it involves a lot of thinking to figure out how to get the most out of you actual food. Some displays give more candycritters. And I dont want to waste superfood on dragons (mushroom in garden. should I use supershroom and get caterpillars? (could be an idea, if you are doing a scrollsquest of the rainbow at the same time) or avoid mushroom and have mice and mallards? I think that i find that the orange Cherrymole is harder to get to visit than the red, yellow and blue critters. Is it so, that the animals in the correct colour is more rare visiting than the wrong-coloured?