When using a cold frame, one generally needs maximum sunlight and maximum heat, therefore south is the best direction. But keep iin mind, that it is usually 'cold' when using a cold frame, like in the 40's or 50's, or colder depending upon what crops are being grown.
If the cold frame is being used for some warmer weather purpose, then east facing, with shade to the west may be the better option. Also, on warm days, nearing 70 degrees or better, the cold frame must be opened, or the plants may cook. Some cold frames have an automatic opener.
Eclectic gardening style, drawing from 45 years of interest and experience. Mostly plant in raised beds and containers primarily using intensive gardening techniques.
Alex
Cold frames have caused me more heartbreak than woodchucks rabbits, and hornworms combined. Yes, facing south is the correct orientation. Automatic control is critical. It only takes a couple of hours for the solar cooker to ruin weeks/months of work. When they work, the benefits are awesome...but when things go bad, they go bad fast.
I started a compost pile, because I gardened. Now I find myself gardening, so I have someplace for my compost!!
I have had good success with my c old frames, but they do take some managing, or as someone said, you can cook your plants.
Mine are up against a building and face east, getting some Sun until about 2:00 pm.
I use mine mostly for perennials that I've started in the greenhouse, then move when large enough. also, I use the cold frames to overwinter some potted perennials.