Took about 5 years for Onyx to release a color e-reader. People are buying them. They are kind of niche because the screens are greyer than monochrome screens, and the main use of an e-reader is to read b/w text. But it's first gen tech. As contrast improves, more color e-readers will be sold. Onyx is always the first to push the new tech. Eventually Amazon etc will start releasing them. Give it about 3 years.

there are some ereaders with color already but they are niche because there isn't a huge demand of color.

I think they are niche because they have a much worse reflectivity (and hence contrast) than usual monochromatic displays. The existing approaches use sub-pixels to create color via additive color mixing. This approach throws away roughly two thirds of the incoming light. E.g. a red pixel consists of red+black+black sub-pixels.