I'm afraid I'm not entirely sure what your problem is. I'm assuming that you are looking to change the color of the glass on a light bar? I really only have experience with non els light bars. I also don't think there is a universal blue color used by all creators so that is probably a dead end.
Assuming you are using a non els light bar and are using gimp to change the color of glass to blue, you should probably start by erasing all the colors of the light bar to have a clean fresh area to paint. Pick out the blue you want, set the opacity down to about 50 and paint what you want blue. Do the same for red. I'm not an expert on GIMP but I do this so I don't get weird amounts of opacity in different places which can happen sometimes without me knowing.
Here is a light bar I modified to change the white to blue. You can see that you can see the default checkerboard pattern through the colors.
If you are changing the red to blue, on non els light bars it will still show red but it might be purple if you changed the glass to blue and its still shining red. I don't think you can change that.
That is my take on your problem. Hopefully it helps some. I have a feeling that you had a different problem and I just went off on a tangent though. (Oops!)
Old S-10's, Ford Rangers, dump trucks, full size trucks from the big three. You name it, its been used in some way by departments all over the states.
If your looking for a ford ranger to use...
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:P
You're work here is really top notch! Love it all! Maybe you can help me get this model into the game?
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If not, no biggie! Keep up the good work!
This is a model I edited to include a interior and factory correct Alcoa wheels. I've gotten frustrated with trying to optimize it for GTA 4 so I'll just upload here as a standard model for all to use. So hopefully someone will see potential for it optimize it for GTA 4.
Interior was hand modeled by me so its a bit crude (AKA is sucks) along with the wheels. They are modeled after factory Alcoa wheels that came on FX4 trim levels. This was originally going to be just a personal file for me (hence the bad interior) but since I can't figure it out it shall be a free model unlocked model to the community. If anyone does anything with this model I hope they leave it unlocked as well for all the modify and enjoy.
~LoneWolf