A Toby is more effective as a fish taker if you polish the back so it shines brightly. Otherwise stick a strip of silver prism tape on the back. The reason you don’t want to paint the back is because there wouldn’t be any “flash.” As the lure is retrieved through the water there is an intermittent flash as each side turns over. It is this flash that triggers a strike.
Try this. Hold the head of the lure between the finger and thumb of your left hand. Place the forefinger of your other hand on the tail end of the Toby and move it from side to side (at the rate it would be when retrieved). As you do this look at the lure from behind. This is the view that a following predatory trout would see. The lure appears to be alternating between its darker side and its flashing back to very accurately simulate the swimming action of a small bait fish. Painting the back would ruin the affect altogether.
I have used Toby lures for many years. The prism tape on the back really improves your strike rate. A Toby, when you compare it today with the ultra realistic lures available, looks unlikely to fool a fish into a strike. Nothing could be further from the truth.
Here is my tried and tested method of “improving” my Toby lures. After a while all the paint wears-off especially if you fish on a stony bottom. I paint the convex side gloss black and stick a strip of silver prism tape on the back concave side. When the black paint is dry I stick narrow stripes of chartreuse (yellow) prism tape across it. This colour scheme works very well on trout and landlocked salmon in Lake Coleridge and is excellent for searun brown trout in the Waimakariri River. I have caught trout all over the South Island with this lure.