They might be referring to the brightly colored structural pieces. I got a batmobile, for example, that has a lot of vibrant colors for internal pieces even though the exterior is entirely black and grey. The bright colors are so the instructions are more legible (you can see what was added more readily) and can be surprising when you first open a set, but they’re fully covered in the final build and you would only know they were there if you built it yourself. The giant Imperial Star Destroyer is a great example of a model with a lot of internal color.