1. Start with an image of your choice, for example this nice flower.

2. Try to mask the flower. In this case I would take the Auto Mask Tool from the Tool Bar.

3. Setup the Tool Options to about 30% Tolerance, select Fill Holes and set Soft Edge to 0.

4. To create the automask click with the mouse into the image, try to reach a point in the yellow area that will fill the whole flower (you can expand the automask whenn you press SHIFT and click once more again - to contract press CTRL).

5. Select Tool/Color/HueSaturation. Select the Colorize button ant try to set your color with the Hue slider. Press OK, when the preview is fine. Then press CTRL-D to delete the lasso.