Their color seems to change with mood. I tried to start a extreme bb tank recently. I moved only the best ones to this tank and over the next few days their color would decrease dramatically. Once they are settled in and happy again the color would slowly return. (Very slowly)
Normally you can tell at a very young age if a shrimp is a extreme bb or a shadow panda but it is true that sometimes they dont show their true colors until they start to mature. If you keep black pandas and bb together eventually all the pandas will be shadow pandas. This is what happened to me anyway. All my pandas will show at least a little blue eventually. I kinda miss the all white and black pandas now.