True Lemon Blues are the result of mating a blue based bird carrying the gold gene with a Brown Red (gold birchen) bird. The real key is to produce gold laced blue breast feathering.
The reason I say true Lemon Blue is that some alleged Lemon Blues are really Blue x Buff crosses. These will lack the gold laced feathering. The gold laced feathering comes from the birchen genetic make-up.
The most productive mating is to use a (blue) Splash carrying pure gold with a Brown Red which should produce 100% Lemon Blues assuming the Splash is pure for the gold gene.
Blue x Brown Red = 50% Brown Red 50% Lemon Blue
Lemon Blue x Lemon Blue = 25% Brown Red 50% Lemon Blue 25% Lemon Splash
If you use a Splash Rooster that is gold and Silver carrier, the %s of Lemon Blues drops considerably because 50% of the offspring will get the dominate Silver gene and therefore be either pure Silver Blue females or Silver Blue males carrying both gold and silver. I think the % of Lemon Blues from these matings is something like 12.5% which means pretty low odds of getting the Lemon Blues desired.
The task is to find good quality Brown Reds and a blue based bird that carries the gold allele pair. Unfortunately it's near/is impossible to identify the gold carrier blues from silver/gold or Silver pair carrier by sight.
I know - clear as mud.
Dave