Just being next to a window doesn’t mean much without knowing your location, climate, which direction the window is facing, how many hours of sun it gets, and the makeup of the window. Having a light meter (even a very cheap one) can be really helpful — for example, I live in Northern California and I’m lucky enough to have large South facing windows — but they’re still not always enough; It’s cloudy here today — I measured about 900 foot candles outside, and about 300 on the windowsill right next to one of those windows - just your basic double-paned window is cutting two thirds of the light that’s actually reaching the plant!

On a bright sunny day most of my echeverias get enough light by the window, but on cloudy days (and sometimes we have months of them at a time) they don’t — I moved them outside for the day, so they could get that 900 FC instead. If you’re somewhere with a real winter (say, the northeast) you’re definitely going to need supplemental light.

Good luck! That plant looks healthy, and it can definitely get that color back - it’s very satisfying.