The spirit is the same.
First create a global color variable:
color circleColor;
void setup() {
...
}
Then, let’s you want a red color when pressing the ‘r’ key. You would write something like this:
if (key == 'r') {
randomCol = false; //We don't want random colors anymore
circleColor = color(255, 0, 0);
And of course in draw() you need to change to filling color to match that color variable 