In Text Readability in Colour you can see how readable text in one colour is over a background of a different colour. A small barchart gives a programmatically-calculated guess, which is pretty accurate. Working this out depends on which formula is used for calculating how bright a given colour is, readability is a feature of relative brightness, hue while often referred to as key is only marginally significant for readability, see my page Equivalent Colour Brightnesses. Relative brightness is by far the most important indicator, and size of text makes a difference too. While font size is significant, which font is used seems to be of little relevance, as you can see by selecting from the wide selection of fonts in the Google Webfonts in the dropdown list – you need to give this a little time to load, it will all be correctly filled with patience.