Ok, we're all familiar the with the "colourize" technique, it basically gives monotone colouring & honestly it looks crap... So I'm going to you how to quadtone colour, we're using 4 colours instead of one.
First of all here's an example of the "colorize" technique. I'll use my sig.
And here's it in quadtone. lol, not that much difference...
1. Goto image>>mode>>greyscale.
2. Goto image>>mode>>duotone. Set the type to quadtone.
3. Leave the first coluor as black & set the other 3 colours to reds, or blues, whatever you want.
4. now goto image>>mode>>RGB & we're done
A better way is through "colour balance" (ctrl+B'but thats a little complicated to explain... Just mess with it...