I've had crusts that look different when using different methods. I think if you don't smile or stretch the lips at all the scabs are stable for longer periods and they get yellow because they are drying and about to fall off and the parts of the scab that detaches off the lips start to die off. Also food residues and toothpaste has changed the color of my scabs. After I started leaving it alone and not stretching my lips the area that has scabbed up has gotten larger. My theory on that is when I was smiling earlier and stretching the lips my scabs only covered half of my lip because the surface area was being stretched out wider.
Scabs that fall off after 1 or two days don't have the time necessary to start turning colors because they aren't on your lips long enough. Also some people put products on their lips which will change the appearance of the scabs further.