We can also turn our ball different colors. Changing the value of
ctx.fillStyle will change the canvas' current color; we can set its value to a hex
string of the format '#rrggbb' or to a string
'rgba(r, g, b, a)' where a is a value
between 0 and 1 representing the transparency of the color.
Play around a bit with the colors and the rectangle's alpha value and see
how the objects respond.