What do you want the resulting image to look like? Do you want the brain to render as gray, with the tumor shown in the flame-like colors? If so, you can follow this general approach:
1. Convert the brain image to RGB, using ind2rgb() with the original (gray) colormap.
2. Convert the tumor image to RGB, using ind2rgb() with the color (hot?) colormap.
3. Use logical indexing to identify all parts of the tumor image that are black (zeros).
4. Replace the brain image with the tumor image for those given pixels.