setImageDrawable

        colour.setOnClickListener(new View.OnClickListener() {
            int i = 1;
            @RequiresApi(api = Build.VERSION_CODES.LOLLIPOP)
            @Override
            public void onClick(View v) {
                cap.setImageDrawable(Drawable.createFromPath("res/drawable/cap"+i+".png"));
                i++;
                if (i==9) i=1;
            }
        });