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. The nature of the Mirror Lake is the same as of the Rusty Lake.
In other words - it's fueled by dead people's memories. All of the characters who died in the lake and ended up in the Mirror World are lacking their memories - the lake has taken them away. Utsuro's mad spirit looks and behaves too much like a Corrupted Soul and the 5th ending basically confirms it as it uses Mishiro's negative memory to drive her mad and turn her into the same thing as Utsuro.