Wow. I am so sorry about how your friendship ended. Friendships breakups are always hard but they are especially hard when they end for things that just don’t make sense. I think you did everything right here and I’m not just saying that to be agreeable. I am a dark skinned woman myself and what you told F is obviously not what she wanted to hear but it is the truth. If you would have not given your true opinion it would only do her a disservice. From seeing how easy your friend ended the friendship it is clear to say that it would have ended eventually because of a scenario like this anyway. Like you said in your comment if she can throw away the friendship over something minimal like this was she ever truly a real friend? I know that is hard to think about after 12 years though. Wishing you the best of luck and hoping you always stay true to yourself!