"only XL-3"

Please provide a clear explanation of the rules for trading colors! I don't have much experience with this, and I haven't received any specific suggestions yet: let's be clear and concise!

What's so complicated about it?

First, mark the templates, then start the bidding. The sequence of marking is 1-2-1, meaning the first player marks 1 template, then the second player marks 2 more templates, and then the first player marks 1 more template. The order of marking can be determined by a coin toss (in my opinion, it doesn't really matter which order you use).