Adding to the previous answer, you can get color variations from Wisp, tree drops (2 on your island daily, 1 one each NMT island), balloons and islanders' gifts.
I don't know if it helps, but selecting the "Customisation" option on a workbench lets you see which items on your pockets can be customised, even if you don't have the kits/materials with you. The catalog on your NookPhone shows a little label on the left side for items that have either color variations/custom variations but misses the items that can be customised with your (or others) designs- this omission is not present on the catalog app of the NSO app. The NSO Catalog has a filter that shows every item that can be customised and also has the little labels for the color variations.
For reference, the game has 1,142 pieces of furniture as of 1.8 update. Base NL had 1,271 and amiibo'd NL had 1,764.