Fish Shell: How can I customize the colors for the autocomplete feature?

I found it, after checking these docs, and doing some trial and error with different environment variables that might be responsible for that feature. The environment variable is fish_color_search_match

and the solution is changing the variable like:

$ set fish_color_search_match --background='333'

Where '333' is the color code. It is also possible to use predefined colors like cyan, green, blue, etc. that are available.

Also, I'm aware that maybe this belongs better in Unix&Linux StackExchange (which might have been the reason for the downvote?)

Edit: Take into account that colors will not look correctly if your terminal is not using 256 colors.

Adding to top voted answer. The command fish_config colors has been removed and you should instead use the following:

$ fish_config browse

it's more convenient to setup all color configuration via fish web UI, which can be done from cli:

fish_config colors, which launches the config server of fish, and there are already tons of predefined color suggestion

For more info, check out The Fish Shell