Color EInk displays built using a grayscale display and color filter are already available, but sport lower contrast and a bit washed out colors. [1]
True color E Ink displays with multiple colors in each pixel were thought physically impossible, but first prototypes were shown last year. I suspect we're relatively close to a production version.
1 I'm a cofounder of a company that develops and helps develop e paper products and we sell color E Ink development kits. https://www.visionect.com