I had this idea today to sort by image colour in the lightable view, so choose “mostly red” or whatever then get the mostly red images…
You will need some image analysis program to inject the tags for you.
In some apps, one can extract the image hues as a grayscale image where 0 to 255 = red thru green thru blue and back to read. For example, pure green is quite close to 85/255.
As to tagging in dt according to grayscale values, I can’t help - I don’t use it.
There’s a contributed Lua script that may do what you want, but it could be an overkill: it analyses and tags images. The names of colours is among the provided tags.
https://darktable-org.github.io/luadocs/lua.scripts.manual/scripts/contrib/photils/
thanks, I’ll take a look 
I am in the process of writing my Bachelor Thesis exactly on this topic.
Spoiler: I have some methods that work way better than expected and are (by design) primitive enough to be used anywhere where a thumbnail is available.
Once this whole university stuff is over and life has settled down a bit I have the plan to take a look at how to implement this into something more usable than a scientific prototype. I chose the topic of the thesis for a reason.
But as they say: don’t hold your breath.
sounds interesting, it was the focus peaking which can also be switched on in the lightable view that gave me the idea, for some reason I thought it might be possible to use some existing code in some modules to find colours and so on