Flash and Stun mods tend to behave erratically. Re-verify your MW3 game cache. Then install House's Chromatix Texture Hack (the link is below) and tweak it to how you would like. There are far more options and ways to give you a tactical advantage with that release anyway. Flashes and Stuns are rarely used properly outside of TWL and MLG matches and in Pubs generally just give away the position of whoever tossed them. Unless you drop that flash/stun at someones feet or they are staring at it close range, it will just give away that you are coming in.
Below is House's Release. Be sure to thank him. It is an unbelievably well done release that still works flawlessly (Even with DLC) and can help everyone, especially someone who is partially color blind like me =P
If you want even more of an advantage put UAV on your secondary weapon and tweak your classes with Jorndel's awesome (and official =D) MW3 Mpgh Trainer. Again be sure to thank him, people like Jorndel and House are what keeps our community strong.