I think the grey looks nice on the coral, but if you want to change colours go for it. If you are practicing on a quilt, matching thread colours is far more forgiving than contrasting colours which can show any little hiccup. The other consideration, is being able to travel from one area to the next with your colour choices in thread, because what is really a pain is having to start and stop all the time. Your diagonal quilting fits the bill, unless you plan to cross hatch, then you're in trouble
. I've changed thread colour on my quilts for various reasons, it's all personal preference on what you want to see
