I had a potted red color Bougainvillea with variegated leaves. As soon as I planted it in ground it has changed its color to purple. I have given it the same amount of sunlight and water. Can I add anything to the soil to get its original color.
Most all Bougainvilleas that are grown in pots in nurseries are hybrids and they are quite fragile. For that reason it is not generally a good idea to plant them in the ground. Even repotting is not a good idea. It is best to keep them in their original pots and place them outside in light shade, but not direct sun.
I am not familiar with your local climate, but my educated guess is that it is the increase in light intensity that has caused the flowers to change color. Outdoor light is many times more intense than indoor light even though it may appear the same to our eyes.
Exposure to much colder temps can also cause flower color change.
There are no soil additive or other instant fix for flower color. You may be confusing this with Hibiscus that do change color depending on the acidity of the soil. Stability of environment is what Bougainvillea requires.