A structure which allows players to craft and mix paints is something that I believe many would enjoy.
The ability to paint structures could offer players a great deal of freedom in being able to decorate their bases.
Example of some painted stone walls:
I also think a lot of people would enjoy a new "skin" type such as "Hair Dyes", as unfitting for the DS artstyle as that idea may be.
If one could dye specific parts of their character's hair, that would allow for some interesting color coordination with outfits.
Concept of Lureplant Wendy + Hair Dyes: (not at all related to color coordination)
Being able to apply pigments to equippable items (lanterns, eyebrellas, krampus sacks, etc) may also be enjoyable for some, although that may be very hard to implement.