I expect the reason is stress. Flowers can change colors or fade when they are stressed and transplant shock is certainly a severe stress, not to mention that it was already very stressed before you transplanted it. So if you can nurse it back to health, it will probably come back.
Other things that can affect the flower color:
Pigments in roses are UV sensitive especially yellow pigments but most colors shift with time and continued exposure. Some red or pink roses fade when exposed to too much direct sun, due to breaking down of the pigment exposed to UV rays. Strong colors in complex combinations show the greatest variability as tiny changes in one of the rose's pigments can shift the perceived color quite a bit.
Another impact can come from the soil. Certain minerals will affect color shifts if they are not present. Magnesium is one of those. "Magnesium (Mg): Crucial nutrient that promotes dark green leaves, intensified flower color
, increased flower production, and can also help flush harmful salts through the soil. Thatâ€™s why Epsom Salts (a form of Magnesium Sulfate) is a time-honored secret for rose gardeners. Apply at the rate of 1/3 to 1/2 cup per plant at the beginning of the growing season." http://www.heirloomroses.com/info/care/ ... fertilize/
Some people claim alfalfa pellets have helped roses achieve stronger colors. I don't know if there is validity to this, but it seems pretty harmless to try.
"Why is alfalfa good for roses? As a fertilizer, alfalfa is 5-1-2, providing a good source of nitrogen, calcium, iron, magnesium, phosphorus, zinc, vitamins (A, D, B1, B6, E, K and U) and triaconatol, which is a fatty acid growth stimulant. Because of these beneficial components, alfalfa can provide roses many of the substances that they need to grow as well as stimulating growth.
Alfalfa can be used in many ways. Alfalfa meal can be worked into the soil around rose bushes. Normally one cupful per large rose bush and about one-half cup for miniatures is recommended. Alfalfa meal is also contained in many commercial organic products (e.g. Mills Magic Mix) because of its beneficial components. Alfalfa pellets can be used instead of the meal by again working the pellets into the soil around roses. The pellets will soon break down with watering or by rain to slowly release the trace minerals, triaconatol, and other important nutrients." http://www.rose.org/rose-care-articles/ ... f-alfalfa/
That is from the American Rose Society, so I assume they know what they are talking about.
Here's another article about alfalfa and roses: http://www.alfalfasecret.com/alfalfa-roses.asp