Autochrome is a little confusing and I didn’t do a great job explaining it.. here’s a good writeup from this website:
To understand how Autochrome plates work, it helps if you think of the black-and-white emulsion that’s behind each of the color dots. After exposure in the camera and processing, that light-sensitive layer becomes a positive transparency. Every place where light struck the emulsion during exposure becomes a clear spot, and every place where no light came through is opaque black.
When making an Autochrome of a red apple, for example, the red light from the exposure passes through the orange-red dots but is stopped by the green dots and the purple-blue dots. So every place there is a red dot the emulsion behind it ends up clear, while it’s black behind the green and violet-blue dots. When you hold the finished plate up to a light source, light passes through the clear spots and the red dots in front of them, and you see a patch of red light. But light is blocked by the black emulsion layer behind the green and violet-blue dots. All you see is the red of the apple.