I tried all the "secrets of the trade" and they all turned out to be pure bull****...and a giant time-waster. These included soaking the pickguard in dark tea overnight, or soaking it in instant coffee grounds, or rubbing soil on it, or rubbing dampened coffee grounds, and probably a few other tricks that I had forgot.
I wanted to stay away from tinted sprays because, in the end, they do look like a tinted spray on the plastic. However, this is pretty much the best way to do it short of buying a pickguard made with a cream colored plastic. Wich is not feesible for the home relicer like you and I.
That being said, the tinted spray cans work fantastic. The entire process takes approximately ten minutes from start to finish. Just remember, it's not idiot-proof. If you want it to look real you need to use the spray can accordingly.
One hint I can give you is to spray from about 24 inches above the pickguard so you "mist" it rather than actually spray it as directed on the can. Also, to get that "lighter under the strings" look, get a peice of paper or plastic or something and cover the area where the strings go. But the trick is to have this peice of cover about 4 inches over the pickguard. Maybe get a long peice (three feet) and rest it on some board on either side, then put the pickguard under it. Then spray. A little spray will get under the cover, wich is what you want so you don't see a glaring edge where the tint stops.
After you do this, remove the cover and spray again. This method will allow the area under the strings to tint, but not as much as the rest of the pickguard. Giving you that cool and more authentic look.