Recolor tool to the rescue!   1. Set Primary color to the color you want to end up with.   2. Use color picker tool  + right click your image to select the color you want to replace. This color will be installed as the Secondary color (which is why you used the right mouse button).   3. In the Toolbar, set the sampling mode to Sampling Secondary Color       3. Wind the brush width up to something large, so it won't take forever to cover your imag