Disable color picker in UI when color is being read from the model texture

[jstewart-amd]

There are two ways to color the hair/fur strands in the viewer:

1. Hair/fur is a single color from the color picker
2. Hair/fur gets its color from a texture

When color comes from a texture, the color picker should be disabled in the viewer UI, because it does not affect the color in this situation, and having it accessible when it won't change anything is confusing.