I know there is a mosaic virus that creates streaks of colour in your tulip petals, but I've never heard of completely changing colour. The streaked tulips can eventually revert back to red. I couldn't find much info but I did found these:
"It is one of those things which is very hard to explain. Both tulips and daffodils are very strong colored and will not change color depending on sun exposure, moisture, temperatures, or soil pH."
http://en.allexperts.com/q/Bulbs-709/Tulip-Problems.htm"Tulips do degenerate over the years often reverting to plain colors. If it is within your budget, planting new each year is best. "
http://members.aol.com/valecroft/maygardeningqanda3.html"Q.
I don't get it. I have lived in my home in northern Idaho for 5 years. When I moved in there were no TULIPS on the property. I have planted various tulip patches, mostly in pinks and purples. A couple years back I notice some red tulips and some yellow tulips appearing.
At first, I just thought they may have been old bulbs that were for some reason dormant and had been "stirred up" by my planting, but each year more and more red and yellow are appearing and less and less of the colors I planted. Are my tulips changing colour? I planted three tulips in a certain corner of the garden and they were supposed to be a pale greenish white
flower, which the first season they were, now this year all three are yellow!! I don't get it. Is it some sort of pollinating thing or what?
A.
Your tulips are almost certainly cross breeding and seeding down, the young tulips slowly reverting to basic colours. Some tulips can change colour due to eelworm attack, eelworm is endemic in some soils. If you don't want other colours to appear, remove and destroy the
seed heads after flowering. "
http://members.aol.com/valecroft/maygardeningqanda.html"Highly bred tulips can revert to parental forms, or get a virus transmitted by aphids, which cause color variations. "
http://discuss.washingtonpost.com/wp-srv/zforum/01/garden041201.htmI have heard of people who use tulips as annuals and just discard them and plant new ones each year, but my wallet doesn't allow me to do it. Besides, they come back year after year and the varieties are the same offered every year. Hope the quotes and links helped a bit.