I don't think you can? SelectionColor uses the ansi escape code for black on white: "`e[30;47m" (ps 7 for the `e). https://en.wikipedia.org/wiki/ANSI_escape_code#Colors
How about this from that wikipeda page:
ESC[ 38;2;⟨r⟩;⟨g⟩;⟨b⟩ m -- choose RGB foreground color
ESC[ 48;2;⟨r⟩;⟨g⟩;⟨b⟩ m -- choose RGB background color
Red foreground (255 0 0) blue (0 0 255) background.
Set-PSReadLineOption -Colors @{ variable = "`e[38;2;255;0;0m" + # fg
"`e[48;2;0;0;255m" } # bg
In ps5 you have to say $([char]0x1b) instead of `e.
$e = [char]0x1b
Set-PSReadLineOption -Colors @{ variable = "$e[38;2;255;0;0m" + # fg
"$e[48;2;0;0;255m" } # bg