What exactly do you mean by colorize?
*To make the whole banner the same color:
I [still] use PSP8... my "old" way of colorizing was making the full banner, merge to a single layer, duplicate, then would do the old "shift+L" to bring up the "colorize" box. Then you can pick the color and intensity, then apply. If you don't want it fully monochromatic, you can take the upper (colored) layer and set it to screen, overlay, hard light, or soft light. That way the tint covers the whole banner, but the natural colors are still visible too.
Does that even make sense outside of my head?
Another awesome thing is VirtualPhotographer, which has some excellent effects (including tinting banners, etc.). You can download it here for free: http://www.optikvervelabs.com/virtualPhotographer.asp
Honestly, a lot of making a good banner is just messing around with the settings until you get something you like.
*If you mean having a picture and then only having the yellows, or the greens show, you can use the "shift+H" to bring up the Hue/Saturation box, then on the down arrow pick the colors you don't want to see "yellow, reds," etc, then take the saturation slider all the way down to -100.
