Use the arc() function to create the rainbow below. For the arc function you need six parameters arc (x, y, x, y, ___,___). The first two parameters are the center/bottom of the arc, the third and fourth are how wide and how tall, the last two are taken from the measuring device picture below. Where you start and where you end. For a rainbow you start at 3.14 and end at 0.00.
For this particular sketch, the first two and the last two parameters do not change, only the middle two (how tall and how wide).
Also, you need to use noFill() or each arc will print out over all previous arcs.
You will need the measuring device below to figure out the last two parameters.