"It's interesting to learn any color is permissible. Are there any "rules," per se, to be observed when visiting Mecca?"
When my parents did Hajj a couple of years ago, my mother wore a variety of different coloured clothes because there is no such restriction. For example the Indonesian women tend to wear white, (which makes alot more sense) and alot of the African women would have the traditional birghtly coloured dress of their countries.
"For women, they must have a mahram, a male relative or husband, accompanying them"
To clarify this can only be a male relative that a woman cannot marry, eg. her son, husband etc.
"OR they may travel with a group of trustworthy women (their respected elders preferably) for safety reasons. "
When completing Hajj, the most important pilgrimage and the one every muslim should complete, women are not permitted to travel alone. The only exception to this is if she has no male relative (as explained before) to accompany her. Then she may travel with other women who in the same situation.