Answer: Cotoneaster can be grown in full sun, however it will be more sensitive to dry soil when grown in more sun. New plants in particular will need careful watering the first few years while they become established, later on they are more drought tolerant. They do best with evenly moist yet well drained soil, so it does not need to be sopping wet/saturated, but it should not dry out completely. Using an organic mulch can help reduce watering needs.
Based on your description, the foliage could be showing the effects of dry soil combined with sun, or there might be a pest problem such as lace bug or mites. I would suggest you consult with your local county extension to obtain a more specific diagnosis of the discoloration and based on knowing that, decide what to do.
Q&A Library Searching Tips