Long post incoming, sorry!
The pattern tool is kinda weird to work with, and it does take some practice. Part of that is because what you draw on the right doesn't directly get put on the shirt to the left, the game tries to 'help' you a little. Sometimes it works in your favour, sometimes it's the worst thing to deal with.
It will try to find straight lines and curves, and smooth them out. The way it does this can make it really difficult to draw things of similar colours next to each other. Knowing that can already help sometimes! Here's the kind of behaviour I'm talking about:
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I've quickly drawn a shirt as an example. It's by no means perfect, but it may help a little?
You'll want to avoid overly bright or dark colours, they'll look a bit odd on your character. You're looking for something like the difference in these two images:
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I've seen people not know this before so I will mention it: the "change palette" tool is your friend for this. You can set every colour individually if you want! It's the little tube thing.
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Adding a few colours that are close to the other ones you used will make a design feel a lot less flat:
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You'll see a lot of designs use more than one colour even for solid areas too. It's how you draw rougher fabrics, and can make a design a little more interesting if done well. It takes ages to get it looking okay, though! That would look a little like this:
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Some final quick tips:
- Finding clothing you like online and trying to recreate that in-game is very helpful if you want something more complicated than a simple printed design.
- If your design has things that stand out like ribbons, buttons and layers, use more colours for those so you can try drawing shadows. It's the only way to prevent them from looking flat!
- Try placing your design down in-game while you're working on it. Sometimes what looks good in the editor will not look good outside of it, there's always a slight difference. Adjust accordingly & repeat...
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Hope that helps!