The light shone on the walls allowing for some of the rune patterns to be clever to Walker. However, it also lets the faint colors that used to be there also be clearer.
As much as any race in ancient times used pigments from flowers or other sources to cover walls or create something beautiful, time would eat away at the color since it was natural in the first place and everything would return to that decay and growth cycle. So when he noticed some of the paint was still there he was curious to see what he could make out. It was different from other ruins he had seen that were only carved in to the walls or had no remnants of color left within them.
