Cold frame - too cold?

So I had OH make a cold frame last year out of some ply wood and a Perspex top. I painted the inside and outside with Shades garden paint (various colours, what we had left from painting the dog kennels) but over winter it has gone black on the inside. It is down the side of the house in the shade. It is on patio slabs with one inch of stones in the bottom. Should I spray the inside with mould spray or wash it down? I assume it being in the shade it is too cold for a cold frame? Or is the idea that hardening plants off that they should be in the cold but frost free?

Thanks.
Tink.

 

I think it's probably gone black due to not enough ventilation.
I have the top of mine ajar even in the winter as long as the temperature is above zero.
I also have a cover of bubble wrap that I drop down over the glass on frost nights.

 

Thanks @pete Yes I did think it must be lack of ventilation. Have only put some Buddleia cuttings and sprouted conkers in there atm as not sure if the black on the sides would harm anything else. Won’t put any veg seedlings in there just in case.

Have left the lid up on its lowest notch for last night and looking at the week’s forecast no frost is showing (yay) so will leave it that way for the foreseeable. Will give it a good clean and airing next week.

Still wondering about the best position for a cold frame, shade or sun?

 

Well I always close mine over night as often nights are damp any way.

I doubt that the black mould will attack plants.

 

Oh really, Close it at night, okay thanks @pete will do. Guess that will stop the slugs too !!