Mine is grey.
Thats exactly as I remeber it.
It is obviuosly a mixture of things there are white crystals and at least on black powder. There seems to be no yellow at all.
The MSDS sheet is very misleading as it only list the potassium Ferrrocyanide and gives the (incorrect) formula for the Sodium salt (they ignore the nitrogen atoms attatched to the carbon [the cyanide group!!])
Reading through my machinery handbook they give a few recipes that use Cynaide for colour case hardenning (usually the Potasium Salt). They also state that if the colours are too rich leave out the cyanide!
So will WE!
I think then you should try a 50/50 mix of
Bonemeal Fertiliser (from a garden centre)
Crushed wood charcoal.
Maybe if you have some Borax (used as basis of silver soldering fluxes) add about 10% of that. This should glassify and help the powders to stick to the metal
You need to pack the metal and the above mix into a loosley sealed box, or better (they say) into a piece of iron pipe closed at one end. Put all into fire at dull red heat for several hours. Case hardening is a slow process. At end take out and quench in water!