Install this version:
emerge -a =app-emacs/rainbow-identifiers-9999
If this version is masked, you can unmask it using the autounmask tool or standard emerge options:
autounmask =app-emacs/rainbow-identifiers-9999
Or alternatively:
emerge --autounmask-write -a =app-emacs/rainbow-identifiers-9999
# Copyright 1999-2021 Gentoo Authors
# Distributed under the terms of the GNU General Public License v2
EAPI=8
NEED_EMACS="24"
inherit elisp git-r3
DESCRIPTION="Highlight identifiers according to their names"
HOMEPAGE="https://github.com/Fanael/rainbow-identifiers"
EGIT_REPO_URI="https://github.com/Fanael/rainbow-identifiers.git"
EGIT_BRANCH="master"
KEYWORDS="~amd64 ~x86"
LICENSE="GPL-3"
SLOT="0"
SITEFILE="50${PN}-gentoo.el"