Critical Surfaces....
An aircraft may not depart for takeoff if ANY critical surface has contamination adhering to its surface.
Any aircraft may depart with a light covering of frost on the underside of the wing.
Frost on the lifting surface (wing) is a no no. Aside from the usual snow that has fallen, there is a problem with super-cooled fuel sitting in the tanks that make contact with the upper surface of the wing. If the humidity is high or there is light rain, mist or wet snow falling, the result is a frozen mess on the upper side of the wing.
There are a number of ways to try and get rid of the cold fuel, most to no avail except for one.
You can try transferring fuel from the center tanks to the wings. In some cases, the center fuel, if any is left, can be used to warm the fuel remaining in the main tanks. This method doesn't always work, but its a thought.
You can try to transfer fuel across the aircraft and back, thereby mixing the fuel from left and right sides. This method doesn't always work either.
The last thing, which usually works is to wait as long as you can and then fuel the aircraft. I've found that the in-ground/bowser fuel to be warmer by several degrees. The addition of the new fuel will generally warm all the fuel (depending on the temperature difference and quantity to be added) and keep it above freezing. Usually the surface frost around the fuel tank will disappear.
Lastly, I wouldn't think of departing ANY station with ANY contaminate.
Anti-ice and de-icing fluids are OK.