Use whatever engine you're comfortable with!
If you don't like any engine, why not make yourself a pastel board game?
Remember to keep things simple, come up with a basic action or short story, you can always add more near the end of the jam.
If you want to jump right into things, or don't want to spend a lot of time thinking of your own palette, there's plenty here: http://colorpalettes.net/category/pastel-color/
The best way to depict a pastel theme is to use primarily light colours, examples of pastel are in these games:
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