I know that fall coloration (reds, in particular) with an auxin overdose during layering.
What kind of layering? If air layering, it might just be a defense response (antocyanins reduce oxidative stress caused by damage) or a more general approach would be the cut off nitrogen supply (if air layering is involved).
When working with insect-pollinated plants like maple (from wikipedia), you'll need a soft paint brush and an isolated area to make hybrids. Isolate the two plants you want to cross, and go back and forth between both parent plants, bushing the flowers on both. If bees can reach them, you'll never be able to tell which branch is pollinated by which tree. Do as many as possible, not every genetic trait is equally distributed. In school we learn that it should be 50/50 from mom and dad. But in fact it's just a giant mess.
Now we're talking traits, keep in mind recessive and dominant traits; the cork bark might be recessive while the red foliage is dominant. Cool, you now have smooth bark and red foliage. Or the other way around; rough bark and green foliage.
It takes another generation to get both traits out in the open, that's why I love annuals and perennials; projects take less than a five years to figure out and to get results. I don't know how fast maples mature and flower, but I
assume it takes more than 5 years. Time enough to figure out which trait is which, just by observing the seedlings.
Not every cross is easy to make, there has to be a certain level of compatibility. This is genetically based and it's usually hard to find on the interweb. Search for Latin name#1 x Latin name#2 to see if anyone did it before you. Otherwise it's necessary to dive in deeper and search specifically for compatibility traits. I'm not sure how compatible maples are to one another. But since there's so many hybrids, I think that it can be done.
Genetic engineering isn't always necessary. Protoplast fusion (in the case of sterility, doesn't always need X-rays) or embry recue cultures (in case of aborted seeds) are pretty solid techniques, they require a couple of hundred bucks and some extensive practicing to master though. The embryo rescue, I'm fairly sure, can be done at home with just 200USD, two hours of practice, and a truckload of patience - meaning you'll have to dissect about 1500 flowers with tweezers and scalpels in a sterile environment, at home. Takes about a week or two to do. One or two plants will make it after a 6 month incubation period. If you do this right, give a plant laboratory a call and they'll hire you. If you see a lily flower somewhere, you can safely bet that there's a 99% chance it originated from an embryo rescue somewhere down the line. The floral business is thriving because of that technique.
If you want to go the easy road, some food dyes can have pretty awesome effects on broadleafed plants. But that's at your own risk, it will stain your soil and your pots and your hands and everything else.
