Different water parameters can cause a snail's shell to grow out different colors. This good be good (i.e. if they moved from poor quality water to good quality water), it could be bad (i.e. they moved from good quality water to bad quality water), or it could be neutral and just indicate that there was a change that was neither good nor bad.
As long as your snail's shell looks otherwise healthy (no thinning, no holes) and your snail is active and eating, I wouldn't worry.