Easycor, HTC lists the transparency colour as {127, 127, 127}
You can find this in the squads link from the home page i believe.
FYI I did mine in Paint 64X64 and 256 colour so Paint will work.
Also Im not sure what the "127" actually is, but I painted everything I didn't want to show white.
Just checked my sqn nose art and it shows up perfectly, is transparent where it should be and shows correctly on all colour schemes of all the aircraft in AH.
Hope this helps
Cheers
RTR
BTW my noseart is in your squads folder under rtrbmp. if you want to look at it. (it's also my Avatar)