A google.com search produced this from yahoo.com:

Flower color changes can be caused by changes in temperature, soil nutrition and/or by viral diseases. There isn't a way to change the color.

Might it be sunlight - or a lack thereof?

Thank you for the comments.
The plant probably gets a similar amount of sun as it did in HomeDepot. It is in my greenhouse with a clear plastic roof, with early morning and late afternoon shade. I put it in a bigger pot with good garden soil and keep it watered. I kind of assume the change is from nutrition but don't know what fertilizer and micronutrients bougainvillea prefers. Could also be ph change I suppose.

It's very pretty. At least you can get yours to flower. I don't have good luck with bougainvillea.

I had that happen to a Poinsettia before...it was red when I purchased it and when it rebloomed the following year it was White!! From the research I did I found out that sometimes because they breed these in so many colors the gene pool gets mixed and sometimes they carry more than one color in the plant and can change colors? It way beyond my comprehension,lol...

I have a bougainvillae that I'm trying to grow as a Bonsai...Here's a picture of it last January when it was blooming...

I have just the opposite I want mine to be white mine had a little pink when I bought it and now it’s mostly all pink how do I get it white again I could not find white Bougainvillea’s in my air

I am having the same problem. I fed my plant with bougainvillea food to get it to flower, but all I am getting is white flowers.

Same problem with mine. When I bought it, it was a mix of pink and white flowers and now it's totally white. Can't understand what changes to make. 😔