My classmates in art school would do this to make grey, but I never tried it myself since I mostly buy the mixed ones and also cos lazy lol. It was only until I started investing in artist grade pigments that I understood how big a difference is with student grade ones and how it makes it so much better if you mix them yourself.

I never really cared for ultramarine before (probably I was also buying cheap oil paint brands as a student) and now it's one of my favorites. XD Thanks for the tip, I should try that. I have all the colors mentioned. :D