Assuming the intervals do not overlap, there are no gaps and they are sorted you use a binary search:
>>> keys = [0.5, 1] # goes from 0 to 1, specify end interval
>>> vals = ['red', 'orange']
>>> import bisect
>>> vals[bisect.bisect_right(keys, 0.464897)]
'red'
>>> vals[bisect.bisect_right(keys, 0.5)]
'orange'