Really? When my tranny blew, the fluid was almost black and it smelt burnt. I was running +4.

 

I suppose if it is extreme. That was one of the big things with ATF4 though. Under normal wear and usage, using color and smell was not a good indication of fluid life.

This is in the Chrysler trans maintenance manual, but it was pulled from SAE paper #982674 which documents the development of ATF+4.

"The red dye used in ATF+4 is not permanent; as the fluid ages it may become darker or appear brown in color. The ATF+4 also has a unique odor that may change with age. With ATF+4 fluid, color and odor are no longer indicators of fluid condition and do not support a fluid change. Use 45K miles or 3 Years as your guide also replace filter and have bands adjusted at service intervals."