[request] Allow coloring the options in Reference List

[asitemade4u]

This one is cosmetic but would put Grist even closer to what Airtable allows. Currently only Choice Lists allow to color each option in the list, not Reference Lists. The discrepancy is weird because Reference Lists are the database-compliant way to go.

[anaisconce]

I find myself wanting this too! Especially with auto-coloring so that I don't have to manually set colors for all entries in a reference list.