Rods or cones

You will decide which photoreceptors work at six levels of light, and which levels give colour vision.

The idea. In very dim light only the rods work. In dusk both work. In bright light the cones work and the rods are not useful. Only the cones give colour vision.

Your tasks.

1. Finish receptors. It gives "rods" for levels 1 and 2, "rods and cones" for level 3 and "cones" for levels 4 to 6.
2. Finish colour_vision. It gives True when the cones are working at that level.
3. Set rods_only to the list of names of the levels where only the rods work, in the order of the table.