When I select a texture for a matte near the top of the textures, it applies directly. When I get nearer the bottom of the pile. so to speak, I must select a texture nine textures away from the one that I wish to use.
See this example of a purple canvas - it only applied when I selected the bright green canvas which was nine textures further along…!
Purple matte selected - it shows Green!
Green matte selected - it shows Purple - which was my desired color in the end!