How to fill the selected UV island with colour when texture painting? I am trying to color parts of the UV map below with the paint function.



My problem is that I don't know how to select the square in the UVeditor so to be able to fill it simply with the desired color instead of having to paint it by hand as you can see here:



My goal is once again to have the upper left square in the UVeditor, which corresponds to a face of the cube, completely colored. 
How can I tell blender to fill with a desired color a part of the UV mapped mesh? I need to have a clear distinction between the colors of the faces and with the coloring procedure by hand this is quite impossible, especially for more complicated meshes...
Go to edit mode 
select the faces you want to paint on 
Turn on texture painting, in the header of this window turn on the button named "face selection masking for painting", an icon with a cube with a face red/white
choose the "fill" brush
clic on a face
done!


Use "shift" key to select/deselect faces without entering edit mode