Amy, I love both blouses on you, with the citron edging out the other a bit. The yellow has a wonderful brightening effect on your pretty face and yet there's a softness with the pink/lavender one, too.
As for the belt, I have black and yellow Halogen skirts and while I wish I could add a belt, it just didn't seem to work with a blouse tucked in. I did wear a black stretch belt with my black one and it worked fairly well, but it wasn't perfect and I kept feeling behind me to see if it had lifted up. So, next time I'd probably go without a belt, unless the shirt was out. I think your new magenta piece will look outstanding with your skirt. You do work in a tempting environment, so just peek a little as you walk quickly by!