Scratch

i'm making a game where the a sprite changes size depending on the colour of the surface that another sprite is walking on. the two surfaces are green and brown. if i move on the two surfaces one at a time, the sprite size changes alright. but I want the sprite's size to change as it moves across from green to brown. at the moment, if i continuously move from green to brown, the sprite doesn't change size. it will only change size if the main sprite stops.
please help    if you have questions or want me to include the script i will but at the moment, i don't think that will help. tell me how you would do it and i'll say if that works.

HOPE YOU GET WHAT I MEAN