Ah, that's easy. You have to select certain parts by using the Free Select tool. Use colorize. Then select the next part and use colorize, but choose the latest used PRESET color to be consistent.
To cut out parts of the image, duplicate your layer without colorization and colorize the top layer. If you have already colorized, put the duplicate as the top layer and click Base Image on the edit history for your original, which is the bottom layer. Use Free Select then click Scissor Select and press the delete button on your keyboard to remove an area, then switch from top layer to bottom layer and use colorize.
