As a WMG subpage, all spoilers are unmarked as per policy. You Have Been Warned.
The artist in the Realm of Beauty is Guertena(Ib spoilers below)Because. But other than that, the paints you need to bring to him are red, blue and black, the first two being the colors of the roses belonging to Ib and Garry, the two main human characters of Ib, respectively. Also, his masterpiece is the painting of an eye - eyes are such a prominent motif in Guertena's paintings that there's a book dedicated to them in Ib. Also also, the music playing in the Realm is the same track that plays in one of the Bonus Dungeon of Ib, specifically the one with the only Guertena's portrait ever.