HL2 Master ... As others have said, dyeing a pair of pre-existing Tiger BDU's is probably the best idea.
Personally, I would use some Tan Tiger BDU's as a Base and then boil them in Liquid Rit Dye "PEARL GREY" .
You might have to repeat the process several times, but I'd wager that this will achieve your desired effect.
Below is a Pair of BBI Kretchet Tigers that were blue.
I overdyed them Red.
Basically, what I should in theory happen if you dye the Tan Tigers with Grey is the following:
The Brown should turn a Dark Grey.
The Medium Tan should turn Brownish Grey
The Light Tan should turn Grey.
The Lightest color will pick up the dye color the most.
I hope this helps.
Best of luck and happy customizing.