One reason why the inside of the Basilique Saint-Julien in Brioude is so colorful and luminous is that new, brightly colored stained-glass windows (vitraux) were put in place all around the church in 2008. They replaced plain clear glass windows, changing the church's look and feel completely, I'm sure.
The new windows were designed by a Korean priest named Kim En Joong, who was selected through a competition organized by the town of Brioude. The church's old stained glass hadn't survived the 1789 French Revolution.
Different colors dominate in windows in different parts of the church, so that the light and colors inside the building change from one part of the day to to the next as the sun moves across the sky.
I only photographed a very few of the windows, but there are 37 in all. Kim En Joong has designed stained glass for churches in Tours, Angoulême, Brioude, and other towns and cities, including the Cathédrale Notre-Dame de Chartres.
You can view dozens or even hundreds of Kim En Joong's creations here.