Okay, step 1 : make the class
class ColorContainer {
color col1=color(0);
ColorContainer(color c_) {
col1=c_;
} // the constructor
} // the class
See reference class and website | tutorials | objects
Step 2 : before setup ()
ArrayList<ColorContainer> listC = new ArrayList();
Step 3
In setup make a for loop using lerpColor
for(int i=0;i<100;i++) {
float amt=map(i,0,100,0,1);// tells point between 2 colors for lerpColor
color c1=color(255,0,0);
color c2=color(0,255,0);
listC.add(new ColorContainer(lerpColor(c1,c2,amt));
} // for
https://processing.org/reference/lerpColor_.html
Step 4
usage is fill(listC.get(3).col1); for example