I think either metal colour would look great, however my personal preference would be to get it in yellow gold. I have a Cartier Love cuff and a 3 carat diamond tennis bracelet, both in yellow gold. I confess I don’t wear the tennis bracelet as much as I should but I do love the look of the two bracelets together.
As for carat size, I would say to get the biggest and best quality you can afford. However, if you are going to stack both the Love and tennis with a thin JUC, the 3-5 carat size range is likely to work the best so that it doesn’t overpower your stack.
Not a great photo but I’ve attached a pic of my Love and tennis bracelet together to give you an idea of how they look. My Love cuff is a size 18 so the equivalent of the full bracelet in a 17.