You may have noticed that a more usable, intuitive flat UI form involves some degree of redundancy. It’s often not just one visual design component (e.g., color) that communicates difference. Instead, it might be color and shape, or color and size.
Jessica Enters, Flat UI and Forms (in A List Apart)
Tl;dr: if you have a flat design, be careful not to let you form elements blend in too much and get washed out.