Mentioning `add-ons` on [geany/geany#2972](https://github.com/geany/geany/pull/2972) caused me to look at it, there is an option to make a coloured tooltip appear over some things that look like hex colour specs. (See what I mean by its a "catch all", its easy to miss things.) The capability is not CSS specific so it doesn't understand names or rgb() AFAICT but it may be sufficient for @82d or somebody could improve it.
I beg your pardon for delaying the answer. there was a lot of work. the essence of the question is to display all the colors at once, and not when seeing. I want it to be like this [plugin](https://github.com/ap/vim-css-color) does.