Caran dAche refills are fairly simple to choose, with either ballpoint or rollerball pen versions. The only caveat is the Ecridor XS range which has its own pen refill that is not compatible with other Caran d'Ache pens.
Any refills on this page not made by Caran d'Ache will still be compatible with their pens, check the product descriptions for more guidance on which ones.