I think Hector's exact words were something along the line of "For some reason I was expecting [insert flower here]." I think it indicates that he thought that whateve her was expecting suited Don's personality. So that would indicate that the flowers are determined by your personality, as you said.
However, what I find more likely is that the different flowers are caused by the sproutella. Perhaps there are many "flavours"? After all, Salvador and Olivia were sprouted with the same sproutella, and they both got red flowers even though one was noble and one was evil. Also, Manny was sprouted by slow acting sproutella and got a blue flower. Lola's death took really long compared to Don Copal (so it's safe to bet that she also was shot with the slow stuff) and she got blue flowers as well.
Or perhaps there is no formula at all and it depends on what mood the animators were in at the moment. Who knows?
And don't worry about your English! Es ist zweimal besser als mein Deutsch!