If you mean on the outside, paints that apply well in vacuum and microgravity probably need to be developed and tested first.

If you mean on the inside, it'd be a lot of time and disruption to devote to maintenance on a station that's already having to spend an increasing amount of time on maintenance instead of science.

The modules have a lot of stuff that has been wired between them over the years, all that would need to be sorted out, consequences understood and more before ever starting the work, and by then it'll be time for the ISS to retire anyway.