First make any of your desired colors and lighting corrections to that original full color image. Colors > Color Depth Reduction > 2 colors, to create your black and white version. COPY the new black and white version into your Windows clipboard by right-clicking the image's titlebar and chosing COPY. Use UNDO to back up to your original full color image Rightclick the title bar of your image and choose PASTE AS NEW LAYER. This installs your newly created two color black and white version onto a new layer, residing directly above your original full color version. Use the ERASER on that new black and white layer, to reveal colorful portions of that image residing below. Go ahead and clean up any other undesirable areas on your two-color layer at this time. File > Export > Gif Optimizer, and save your image as a 256 color GIF at Optimized Octree