I think changes in growing conditions (temperature, level of light, soil conditions - presence or not of fertilizer, or too much fertilizer, plant maturity) are all factors in flower color.
These plants are definitely not 'cross contaminating' each other - that would only apply to any offspring grown from seed from any of your plants that might have gotten cross pollinated.
I see significant changes in flower color for a lot of my named Echinopsis hybrids to the point that some do not look a lot like the flowers are supposed to look (for example comparing the to collection photos from the Huntington ISI Schick hybrirds catalog) anymore. These plants tend to flower multiple times a year and can have distinctly different colors per flowering cycle. So this must be mostly environmental. I have to think something similar is happening to your plants.
I also have Adeniums show changes in color, e.g. white going more pink.
