[Nom1fan]

Can you also confirm that your render method in Kodi's Player setting is set to Pixel Shader and not auto or dxva 

 This changed a lot ! Now in Kodi 18 preAlpha2 with Pixel Shaders instead of Auto, I get very nice colors on "The Matrix".
Although, "The Terminator II" now looks better than on Kodi 17.6, it still looked much more vivid with Render method set to Auto, but you can be the judge. Please compare the following screenshots with the ones from my first post:

The Terminator II - Kodi 18 - Pixel Shaders
 

It's been a while since I've watched it, but I don't ever remember these scenes being particularly vivid, are you sure that is not a correct representation? and the more vivid one is not overblowing the colours like a vivid mode on a TV should never ever be used as it will give you incorrect colours. 

 I thought about that. It might be. It's just that the whole movie looked so awesome to me and many other people when we watched it that way. Just a matter of taste, perhaps.

Anyway I will test on Kodi 18 nightly build and report back. Either way the render mode was definitely the way to go with "The Matrix" which was unwatchable previously. So many thanks to all of you! You've been a huge help! Hopefully this thread will also help other people in the future.