The $2000 dollar question

When I last studied genetics, quite a few moons ago now, it all seemed quite straightforward, with Gregor Mendel growing wrinkly and smooth peas in his monastery garden at Brno.
What I learned then doesn't really help me understand how a single plant, like the Lisianthus in the photograph, can grow flowers of different colours on the same stem! I'm sure that any explanation will be quite complex.
Eustoma russellianum grows naturally in warm regions of the Southern United States, Mexico, Caribbean and northern South America. A number of cultivars are grown for the cut-flower market  where they are known as Lisianthus, Texas bluebell, prairie gentian, or  tulip gentian.