I think it's fine as long as you make it required to have the DLC. The best way I can think of is to actually not make the portrait stuff blue, but to make a blue filter and tie it to a racial trait so that as long as they have the trait their portrait has the blue tinge. Then maybe have an event that triggers only if you have the DLC in question to add that trait to that race.
Only issues I can think of is that the filter might not look as good, and that I'm not sure if the DLC condition works for graphics dlcs
Yeah I have the portraits finished now and I'm quite pleased with the results.
I was just wondering now if it would be allowed to use edited dlc graphics/portraits in your mod while not actually providing the dlc sprites. Just "heavily" modified ones that are no longer useful for people to use as "dlc content" without owning the dlc.