Using black or white text depending on background color
One problem which I encounter time and time again when creating a webpage or applications is matching text color with a changing background color. When creating a GUI you usually create a lot of white boxes with a black text on it, just to get sizes and stuff matching. Then later a designer or customer decides on a theme which affects how that box looks. Most of the time the background and text colors are hand-selected so they match well enough, but from time to time you have to create a box which can change background color on demand, and then you have to have the text color matching to make the box readable....