Help: Nephew's science project

[CapsLockCT]

I recall watching a "home & garden" show which discussed a flowering plant
which either had reddish or blueish pedals depending on the level of a certain
soil nutrient.  Any idea on the plant name? Also, does soil have a specific
nutrient makeup and if so, what are the effects of each nutrient.  Any help
deeply appreciated.  Please excuse my misuse of terminology.  Very "green" here
:)