JarateKingKonceptMy recommendation would be paint.net + vtf plugin or the same plugin for photoshop instead. They just make the whole process a lot easier.
Otherwise, using VTFEdit you can export the image into another file format (tga is recommended but if that's what you want would depend on what you're using to edit it) and when you're done changing it around however you want, use VTFEdit again to turn it back into a vtf.
e: didn't realize they were purely vmt's. Open up the files in any text editor and change the values for:0.0 is the same as 0 (or 00 in hex), 1.0 is the same as 255 (or FF)."$color" "[0.0 0.47 0.78]"
there's a VTF and VMT file for the material that's being used. Does that change what I have to do? Also I'm not seeing a "$color" field within the VMT
"UnlitGeneric"
{
"$translucent" 1
"$baseTexture" "vgui\replay\thumbnails\buff"
"$vertexcolor" 1
"$vertexalpha" 1
"$no_fullbright" 1
"$ignorez" 1
"%keywords" "tf"
}