First you need GCFscape (google it), then go to your steam/steamapps/yourname/ folder through GCFscape and open the "counter-strike shared" gcf. Then go to Cstrike > materials > sprites, select muzzleflash4 (both files) and right-click > extract and extract them to the desktop.
Then open the VTF file in VTFedit (another program you need to DL, google ;)). Then go to export > .tga file and save to your desktop. Then open up photoshop, or any other equivalent program (GIMP, Paint.net) and edit the muzzleflash to whatever (white, smoke I'm guessing) then save it as .tga (not .psd!)
Then go back to VTFedit and import the new file you have created, go to tools > create VMF file first, the original VMF has these selected:
so select the boxes: additive, vertex colour and vertex alpha and save it in your steam/steamapps/yourname/counter-strike source/cstrike/materials/sprites folder. If you don't have a folder of materials then create one called materials, if you don't have one called sprites make one too.. simply click create new folder and rename it to sprites.
Next, after saving your VMF into that location under the name muzzleflash4, save your VTF (texture) to the same location with the same name. Don't worry they have different file types so they will save.
Finally open up counter-strike source and test it out with a gun.
I hope this helps you in some way (I only found out by trying to answer your question now ^^)