I would say a Lampropeltis triangulum gaigeae

tri color as babies
http://milksnakes.com/pictures/encyclopedia/gaigae.jpg
changing color as subs
http://milksnakes.com/pictures/encyclopedia/gaigae2.jpg
solid as adults
http://milksnakes.com/pictures/encyclopedia/gaigae3.jpg


note that its not Lampropeltis getula nagrita (mexican black)



ps. i PROMISE thats the most technical you will EVER hear me...HA!!!