To customize an item, you need to go to the customization counter with the customization kits.
However, I think this reply better suits this question:
Basically the stand mixer (and other items that come in different colors) are seen as separate items in a way. The yellow stand mixer and pink stand mixer both need to be bought separately, and there is no way to customize one to get the other
Aw man, that's super annoying but nice to be aware of. Thanks for answering me!