Color is anything that isn't greyscale. (Meaning it would be added to the category below).

https://xkcd.com/

Most recent comic with color: https://www.explainxkcd.com/wiki/index.php/2849:_Under_the_Stars

List of comics with color:

https://www.explainxkcd.com/wiki/index.php/Category:Comics_with_color

There has been an 8-comic color drought. xkcd comics with color historically come every 3-4 comics with some 30-40 long droughts. Will resolve the moment I hear that a comic with color has been posted or if none has after a month.