Need help with texture files... Thread

So a friend of mine is reskinning the Combine Prison Guards from Half Life 2 for Gmod and we want the eye pieces to be blue like the default Combine Soldier's. However, no matter what he tried he couldn't get the eyes to become blue because the game forces a yellow color onto the eyepieces which means whatever color you add it just combines with the yellow and since yellow and blue are both prime colors you can't combine anything with yellow to reach blue. I was just wondering what it is that causes the yellow color to be added and if it can be changed in some way. Thank you in advance to anyone who helps, any kind of help would be appreciated!