Well the only way to alter the colour they flash in game is by renaming each siren dummy as I said before. They'll either be red (odd numbers) or white/blue (I think your ENB/coronas.wtd or visual settings effect how blue or white they appear ingame) for even numbers.
For the actual glass of the default lightbar, it's not controlled by a texture file. It's painted in with zmodeler. You need to select the polygons you wish to change the colour of (with selected mode on, so it's orange), switch to Vertex mode, then select the paint tool (Surface -> Paint), change the pressure to 100, then open the Colour menu and choose your colour, then hit 'Fill selection'.