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.
Thanks, Tom! I've been doing it a much less elegant way. I guess the only complication is if you'd like strings to count as single values; then you'd need something like `isinstance(foo, Iterable) and not isinstance(foo, str)`. But it looks like that's not the case in this code.