Problem 42613. Convert RGB triplet to ColorSpec string

This is the inverse to this problem. Given an RGB triplet, return the corresponding ColorSpec string. If the input does not have a known string name, return an empty string. A second optional input indicates whether to return the short (default) or long form.

Examples

rgb2str([1 0 0]) returns 'r'
rgb2str([0 1 0],'short') returns 'g'
rgb2str([0 0 1],'long') returns 'blue'
rgb2str([0.9 0.7 0.1]) returns ''