I think we’re confusing two points here:
Alezia’s particles are green because her spread value has a red value of 255.
I agree that interpolating in RGB space isn’t ideal, but it’s not causing the green particles. Maybe we can add an option to do the interpolation in HSL space instead. Not everyone understands the particulars (heh) of color spaces
As for the start/end properties, the main value (color in this case) is used as the middle value for the bezier interpolation. If the start or end values are unset, they match that middle value.