When I replace a weapon model, sometimes that file contains an alternate skin, like to make the weapon darker lighter, or change color and all that, it's like a bitmap image file. Does anyone know where I put these files in order for them to work?

download half-life model viewer, open the MDL file that u want, select texture, select the bmp that u want to replace (make sure its called the same thing, im not sure if that nesscary, but justin case), hit import, select the BMP, check to see if it worked (select renderer) and hit save model (option is in textures)
your welcome