After the portals change, Howl says "See that new color on the dial?" to Sophie, to show her the meadow. He then sets the portal to red (from yellow). Red (the "new portal") was on the original dial - yellow is the new color (replacing blue).
When Sophie leaves her bedroom, her dress has turned from green to blue. However, she couldn't have changed dresses because none of them would fit her after she was transformed (wider, much shorter, etc.).