Answer: It's difficult to say why your soil turns color after rototilling. It might be that you're bringing up some clay or silt and when exposed to air it dries out it looks gray. Try taking a handful and wetting it then seeing if you can form a ribbon by pinching and rolling it between your thumb and forefinger. If you can form a ribbon several inches long, you're dealing with clay. If the soil breaks apart before forming a ribbon, the clay content is low. After tilling, why not take the opportunity to work some organic matter into the soil? Add compost or aged manure and til it in to a depth of 8-10". This will help loosen the soil and also provide nutrients to the roots of your plants as it breaks down in the soil.
Best wishes with your garden!
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