since bash 4.4 you can set variables vi-ins-mode-string and vi-cmd-string in .inputrc. I've been trying to change the foreground and background color of each of those strings by using tput setab number and tput setaf number, no success.
I guess it is possible as I've seen bash-powerline script and it uses exactly those functions to do the coloring.
Any help would be appreciated.
They are not bash variables, but readline variables, so they can only be set in .inputrc, which is not a bash script. You need to use the raw ASCII escape sequences.
set editing-mode vi
set show-mode-in-prompt on
set vi-cmd-mode-string \1\e[32m\2(c)\1\e[0m\2
set vi-ins-mode-string \1\e[34m\2(i)\1\e[0m\2