Single colour images

Hey guys...just wondering...

How do I make an image single colour and white. Like Black and White but where the Black and Grey shades are replaced with blue and different shades of blue or red and different shades of red. So like blue and white OR red and white images...?

I'm sure this is pretty simple but I can't figure it out!

Thanks in advance

Go image -> mode -> greyscale. Then image -> mode -> duotone. Select your two colors. when all done go to image -> mode -> RGB