It's the same with the regular license plate as well, the background of the numbers / letters remains dark at night. I've already tried replacing the license plate characters PNG in vehicle.TXD, didn't fix the issue.
It's a problem with SA-MP, in single-player the letters are written on the license plate texture itself but on SA-MP they added a grey box just over it where the characters are written in. The color is adapted for the vanilla license plate texture and not for ImVehFt etc. You can't really fix this as far as I know.