Sorry for bad English, I used google translator to write this.
Colette and Yome have quite similarities, such as the fact that the trait is minimally similar, at least to me, and the fact that the main characters actually had a reasonable development after meeting Hades in the case of Colette and Elias in the case of chise.
The main heroines of these two series' are adept in medicine. They find themselves working in an unexpected environment whilst using their knowledge as a Pharmacist /Doctor.
(Difference between the two)
Colette wa Shinu Koto ni Shita; is a fantasy shoujo manga while Kusuriya no Hitorigoto is a seinen manga which is more realistic than Colette.
The storis seem to have a similar vibe. They both star hardworking girls in love with a king. The king is a moral and kind ruler. They both are set in an indeterminate place in historical fantasy. The romance in Sabaku no Harem is more suggestive.
These are both shoujo titles set in a similar world. The art is similar. They are both slow romance. Colette is set in a pseudo-greek mythological world, and Ryuu a pseudo-asian. Ryuu features a love triangle and Colette so far, does not.
The heroines of both stories define themselves as a doctor or pharmacist. That is the driving force of their life. They're both sweet shoujo stories with slow and soft romances. Their prospective love interests are both hardworking royalty.