Found the rules. You can do some basic stuff in photoshop. It wouldn't be too hard to get a basically white looking sky from a black and white photo using these guidelines:
Please submit photographs that are un-manipulated and real, and that capture those special moments in time. The world is already full of visual artifice, and we don’t want the National Geographic Photography Contest to add to it. We want to see the world through your eyes, not the tools of Photoshop or setup photography.
Please do not digitally enhance or alter your photographs (beyond the basics needed to achieve realistic color balance and sharpness). If you have digitally added or removed anything, please don't submit the shot. We look at every photo to see if it's authentic, and if we find that yours is in any way deceptive, we'll disqualify it. In case of the winners, we will ask for the RAW files, if available, to be submitted for review.
DODGING AND BURNING: Dodging (to brighten shadows) or burning (to darken highlights) is fine, but please don’t overdo it. Your goal in using digital darkroom techniques should be to adjust the dynamic tonal range of an image so that it more closely resembles what you saw.
COLOR SATURATION: Just as with dodging and burning, your goal should be to make it real. Please avoid significant over- or under-saturation. A lot of photographers make the mistake of over-saturating color, making their images look cartoonish.
SOLARIZATION, MEZZOTINT, DUOTONE, ETC.: These are discouraged as being too gimmicky. There are a myriad of alteration "filters" available in digital photo software; try not to be swayed to use them. They may be cool and fun, but they won’t help you in this contest.
BLACK-AND-WHITE IMAGES: Acceptable
CROPPING: Acceptable
STITCHED PANORAMAS: NOT Acceptable
HIGH DYNAMIC RANGE (HDR) IMAGES: NOT Acceptable
FISH-EYE LENSES: Unless used underwater, they are NOT acceptable.
