Bridget Riley: Fragment 6/9, 1965 (UK). Riley is another artist I discovered thanks to the copious time I've spent exploring the Tate Collection over the past 10+ years, and she also recently wrote a fantastic essay for the London Review of Books about her process and how her work evolved from black and white into color. This is my favorite bit from the piece, though I highly recommend reading the whole thing:
For the last 50 years, it has been my belief that as a modern artist you should make a contribution to the art of your time, if only a small one. When I was young, the situation was very different. Abstract painting hung like a mirage in the desert. The door had been pushed open by a small number of visionary artists – mainly Mondrian, Kandinsky, Malevich, Rodchenko. Although travelling by different routes, each had arrived at what was virtually a common core. Having discarded the figure and nature, what remained? Colour as colour itself, those simple shapes and forms that geometry and writing provided, and the material facts.