The trick to growing amazing begonias is to give them as much sun as you dare without burning them. It is normal for the foliage to lighten in color when the sun is shining on the plant, as long as the regular green color returns when the sun passes. Don't water a dry begonia plant when it is sitting in the sun, the plants don't take up water well when it is dry hot and sunny. The fact that your Chistmas Candy is looking healthy and getting ready to bloom shows that you are doing the right thing and giving it enough - and not too much sun.