I have not done this alot (only once). But as requested shermanfreak here I go.
I recently built a super cobra opting to use the latest grey scheme used by the Marine Corp. However the decals were black and I needed grey. After searching the internet I could not find what I wanted. However I did see materials to make them. So I though I would give it a try. I bought Decal Paper for Laser Printers and Copiers form Micro-Mark ( www.micromark.com ). I scaned the original decals into my computer then I changed the color and cleaned up the image in Adobe Photoshop Elements. Then I saved it to a floppy in several formats. I went to Office Max were I had them print the decals on to the sheet (I did not have my own Laser Printer). I am very happy with the results and will be doing it again seeing how hard it is to get Marine Corp decals. They also have paper for inkjets which requires a little more work than the laser paper. The paper comes clear or white (not to many printers print white).
I hope this helps expand your capabilities.
James Simmons
Lcpl, US Marine Corps