annotate with two different colors based on threshold

I added the capability to annotate an image with features of two different colors. I found this useful for playing around with cutoff thresholds. e.g. see how many features you miss if you increase the minimum size.
It's fine with strings. One of my successful test cases was: tp.annotate(f, frames[0],color='purple'). I added this exception after trying tp.annotate(f, frames[0],color=['purple','green']), and the annotation circles alternated between the two colors-- not a good result. np.size('green')=1, np.size(['green','purple']=2