hi bassna...
you can use both color as follows.
select you brush and choose your foreground and background colors..
then in the top right hand side of the screen you will have a tab called "brushes" - see below

once you have click on this you will open the brush settings..
1 of these is called "color dynamics" see below

play around with these settings...the color jitter settings mixes the colors and allows you to use both.