For openSUSE Factory run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:mkubecek:fun/openSUSE_Factory/home:mkubecek:fun.repo
zypper refresh
zypper install darktable
For 15.6 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:mkubecek:fun/15.6/home:mkubecek:fun.repo
zypper refresh
zypper install darktable
For 15.5 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:mkubecek:fun/15.5/home:mkubecek:fun.repo
zypper refresh
zypper install darktable