Black one is the raw molded plastic. The tan and green they send out for painting, so yeah, extra cost.
Green plastic must cost more
They're the color that they is, thru and thru.
"Cost" has nothing to do with the selling price (on any consumer item that there is).
Supply and Demand has everything to do with the selling price.
And... of course, the economy of scale is a determining factor in selling price as well.
This thread isn't about manufacturing economics, so I'll stop there, with two little scenarios.
I worked at an indoor gun range in the mid 80's, where we harvested brass and range lead, and manufactured bullets and assembled ammunition for range use by the customers. We reloaded someplace on the order of 1.5 million rounds a year, on 7 different automated progressive machine (AmmoCrafter, Star and Dillon). Why did we sell 45LC ammo for $10.50 a box, and 44Mag for $7.50 a box? Because its what the market would bear. Our cost was the exact same.
When Colt came out with the King Cobra revolvers, there were no Wolff spring kits. I bought a King Cobra, and decided to wind up my own springs using SS wire and our lathe. The boss liked, so he said... make some more, we'll sell 'em. So, I made up about a dozen more kits of springs. The boss asked, what are we gonna sell 'em for? I said, I dunno, there's about about a nickel worth of wire in each kit, and once I got going I made up a dozen kits in 20 minutes. How about a buck fifty a kit? Boss shakes his head. He goes, how much are the Wolff kits? I said $6.50. He said, how about we sell this kit for ten bucks, and I'll give you five for every one we sell? I was gobsmacked. Thats more than the Wolff kit, I said. But he said, that's right, but there is no Wolff kit, so they'll pay the price. And they did.