What do people cross to get the "strange" colored brahmas?.
Find them from a breeder. I'm not sure what you are calling "strange".....there are some non-recognized varieties out there that folks have been working on for years. Unfortunately, it isn't quite as simple as crossing one with another and then you've got a different variety.
For instance, I'm working on the gold partridge variety. Some of the foundation stock for that variety in the U.S. came from European stock, a farm called Greenfire imported some several years ago. The partridge is recognized in Europe as a variety. Anyway, they brought over some birds and have built from there. Another group of the variety came from crossing a dark brahma with a partridge cochin. Either way, both lines have problems that some of us are working to get rid of.
@big medicine has been working on laced and a true mahogany for many, many years. I think he uses some Cornish in his crosses, but you'd have to ask him or wait for his reply.
Each time we are working on a special variety, we spend a lot of time trying to "build the house", in other words, trying to make sure that we are keeping the traits of the brahma...the large head, beetle brow, large birds, feather footedness, no vulture hocks, pea combs....and on and on it goes. At the same time, we have to work on eliminating the traits that came from the cross....cochin cushions and bunny tails are two things that come to mind for me. Then, we can work on perfecting the color/variety and pattern that we are working on. Of course, all of this is after studying the genetics and the standards involved ad nauseum and understanding exactly what to expect when breeding two different breeds/varieties together.
So, I guess the bottom line is it is a lot of work and time to try to get a new variety out there that still represents the brahma breed reasonably well. If you are looking for a pretty bird that is "different" from the standard buff, light or dark, it might be best to buy some and then work from there.
Probably TMI, but hopefully I answered your question.