I've only recently started using Vallejo paints, and thanks to Maelstrom Games' 15% off promotions in July I managed to get a nice collection of drab colours suitable for camo schemes.
Vallejo has several different ranges of paints with subtle differences in shade. To help me choose the paints I wanted from across these collections I saved the paint 'swatches' as jpegs in a folder on my desktop. To make ordering and future identification easy, I gave each of the colour's jpegs were a simple name which described the specific Vallejo range such as Model, Game, Panzer Aces etc and then the colour title.
Once saved, the thumbnails are easy enough to move round in the folder to create a complimentary colour palette. You can use Windows' PrintScreen facility to take a snap of this palette and save as a jpeg in it's own right.
When it comes to choosing colours for camouflage paint schemes, I can now simply compare the camo against the Vallejo palette to see which colours are most likely to work.
Again, a quick PrintScreen will capture the palette and camo scheme side by side and can be saved as a jpeg on your desktop for quick reference throughout the project.
You can also use a simple free graphics utility such as Paint.Net with which you can copy the colours from the palette and drop them into the camo scheme (when you have a clearly defined picture as above it helps) to test out different effects.
Now, this doesn't replace trying out the actual colour paints, and I don't pretend it does, but it does quickly give you the ability to try different camo patterns, effects and determine if you need to buy any new paints before starting the project.