Is there a table of RGB values for specific color grading?

To clarify, specific effects such as Sci-Fi, Vignette, Western (yeah, I know these ideas are from Apple's iMovie)

no.

It doesn't have to be a kdenlive specific thing. Just a site that has RGB (R,G,B) values for specific colorings.

Well, in a picture are many RGB values. But that's probably not what you are asking for. But for grading, there's also no such thing like an RGB value for a specific coloring. If it were, you could grade your footage by replacing it with a color clip with a specific RGB value. I doubt you want to do this. Really, I don't understand your question.

But in case you are asking how to reproduce a particular effect, that would be a different thing. Maybe specifically asking for one effect and posting a still of what it does can make others to offer help right on the spot.

For many basic grading tasks the Bézier curves are your friends; these are described as a set of curve points together with auxiliary points. But nowhere any RGB values to be seen to control them. Of course, quite some coloring effects cannot be achieved using Bézier curves, but then we are into specific algorithm territory. Again, usually no single RGB value parameter that could act as the effect control.

Vignette is a single effect that you will quickly find when you take the time to look through Kdenlive's effect repository. Give it a try to see what parameters it has and how they affect your footage. Again, no such thing as an RGB value ... wouldn't make any sense actually.

Western and Sci-Fi may be achieved using Bézier curves.

But ... if all you want to do is achieving some precanned video effects from iWhatever, why don't you simply use this iWhatever? It saves you time. My impression is that Kdenlive is for working on specific grading tasks where a few simple completely canned effects may not be suitable and better control is required. But better control comes at its price: several different parameters to control and adjust to achieve the often subtle grading you need. I wouldn't count Sci-Fi as as subtle exactly.