I totally get what you are saying about your own childhood, and your desire to do better with your own DC.
Someone on MN once posted something along the lines of "is the goal to have a biddable child, or bring up a successful adult?", which really struck a chord with me. Girls, in particular, are often taught to just put up and shut up.
Your DD has really strong emotions, and she is entitled to them. She needs to learn how to deal with them, and part of that is you naming them. Shocker and ledkr have both given good advice about this. Another good resource is the book How to talk so your kids will listen, and listen so they will talk.
In the example of the blue or purple bow, your DD is perhaps frustrated that she has to choose. Name that emotion. "Oh, I can see you're very frustrated that having the blue bow means you can't have the purple bow." Talk it through with her, so perhaps she can come to a solution. Her solution might be that she wears both bows, or that she wears the blue one now, and the other another time, or whatever. But don't give her the solution - let her come to it, herself.
Also, remember, all behaviour is for a reason. If she seems angry, it's because she is. It's just that she's not articulating her anger. Perhaps she's displacing it. She's acting angry about the bow, but maybe she really feels angry that you are now spending more time with her younger sibling, or something else.
Incidentally, "gentle parenting" is not not supposed to be parenting with no boundaries, and anyone that thinks they are doing "gentle parenting" but is really just giving in to their child's every whim, is not following the philosophy correctly.