More to explore:
Pops of color in built in shelving backs.
Love the color on the back of the built ins. Interior by DL Rhein.
Arched bookcase top?
Same color, two finishes - first, paint with matte, then paint "design" in (same color in "glossy").
fireplace with built ins
Metallic wall color.
I love love love the idea of putting a saying above the bookcase - maybe Ex Libris or Bibliotheca - something Latin.