This plugin will fix the dark car colour on Steam version.
Why don't you just use modified particle.txd, I hear you ask. You see, the replacement of particle.txd will not fix the colour of introduction cutscene vehicles. Plus, if you're playing a Russian version it will kill several translated textures. With this plugin you won't need to modify particle.txd anymore in order to fix this issue.
Steam version only (obviously)
Put the ASI file into your GTA III folder.
--Sergeanur, 2015