First off you will want to grab your DTX which you will be coloring. For this example I have chosen the L96A1. One of the easiest guns to mod with and the best to start with.
Now you will want to create a new layer. You can do this by pressing on the following buttons. "CTRL + SHIFT +N"
Now you will want to cover up the area that you want to be colored. For this part I used the pen tool since I'm used to using it. You can use the brush if you feel comfortable with it, since you'll still be getting the same result.
Next you will want to change the blending options of the layer. You can do this by double-clicking the layer and clicking the set of options there. Your best choice is to change it to 'Overlay'.
Now all that did was give it a white overlay which only brightened it up. Now you want to go to 'Color Overlay' and choose the color you want. In this case I chose a blue. Once you have done so you can either set the blending option to 'Overlay' or to whatever you want for this part.
Last up is de-saturation. You can do this by pressing "CTRL + SHIFT + U"
Before
Before :
After
After :
Now we are going to have to save it. Go to 'Save as' under the 'File' Tab on the top of Photoshop.
Then select '.DTX Format' as your option to save the file as.