You can't really do it properly, but this tutorial is good for making stuff look like it's been selectivley coloured.
http://valliegurl.livejournal.com/58163.html#cutid1
If you want it more specific, you can try mucking around with the colour balance (making new layers, uping the colour to full, then erasing all bits that aren't that colour) or fiddle with the curves function a bit. Sorry I can't do much to help!
_________________
<center>
[font=Times New Roman, serif].:Sig by me, open to requests of banners, anicons, avvies and wallies over PM:.
.:For Icon credit, PM me:.
[/font]
</center>
