When you speak of painting the inside of the fork and stays, I imagine you mean the side of the fork and stays that faces toward the wheels. Unfortunately, in most cases, BikeCAD won't display a view that actually exposes that section of the stay or fork. The exception to that would be if you're using a lefty fork or asymmetric stays where the drive side and non-drive side stays attach at different points on the frame.

Assuming you're talking about a scenario where the inside of the fork and stays are not actually visible in the side view, the best thing I can recommend is attaching a note to document your intent to paint the inside of the fork a different color.

This is illustrated below. Also in this image, in the off chance this is what you meant, I've shown the stays and fork painted a different color down the middle of the tube. This was accomplished using the option to show a logo on the seat stays and fork. Instead of typing a word in a standard font, I used one of the cycling brand fonts. Notice that the down tube logo in the example below was created from two characters. One character for the white background and one character for the blue logo. The { character in the CyclingBrands6 font is just a long parallelogram with rounded corners. I showed this shape on the stays and on the fork. I made it even more stretched out using the stretch width field .