Like what you've done. Why yellow armour for the Chaplain?
I've been playing with alternate color schemes for characters like the Chaplains and Librarians. Same as the techmarine, I've made their left arms show the color of the speciality. Just something a little different.
As someone who still doesn't like the idea of Techs and Librarians having such starch different armour colour to the rest of their chapters ever since GW changed stuff up (back in.... what.... 2nd Ed?), I'm also thinking to try out something similar for my own marine characters in my Raven Guard force.