I'm not sure about the Dash 8, but the B767 flight manual has guidance for the pilot :
Takeoff with light coatings of frost, up to 1/8in (3mm) in thickness on
lower wing surfaces due to cold fuel is permissible; however, all
leading edge devices, all control surfaces, and upper wing surfaces
must be free of snow or ice.
Thin hoarfrost is acceptable on the upper surface of the fuselage
provided all vents and ports are clear. Thin hoarfrost is a uniform
white deposit of fine crystalline texture, which usually occurs on
exposed surfaces on a cold and cloudless night, and which is thin
enough to distinguish surface features underneath, such as paint lines,
markings or lettering.
I imagine that the Dash 8 flit manual would have a similar statement.