What is the difference between Red, Green and Yellow Bell Peppers?

The skin on the red & orange ones I find to be a bit more tougher than the green

As regards regular bell peppers, red (or orange or yellow or chocolate brown) peppers are ripe peppers. If green bell peppers remain on the plant, they turn into lovely sweet (and more perishable) red peppers. In my garden, if anyone wants a green pepper, s/he has to sneak it past the pepper cop, who allows only the picking of red peppers. That said, some pepper varieties (like Ace) taste okay when green, while others (like Carmen or Lipstick) are really only decent when red.

There's a nutritional advantage to the bright colors. All those great carotenes, and more Vitamin C than in the greens. I think they cook up a little faster too.

I find the red, yellow and orange peppers to be sweeter than the green ones. They are basically different varieties of the same plant and/or different stages of ripeness.