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.
This is the case I was trying to handle: 
tp.annotate(f, frames[0],color=['purple','green'],split_category='signal',split_thresh=10)
I see now that it is broken. What is the proper way to check if something is iterable?