Include the characters you decide are present, what key you used, chemical test results, sketches, etc. The hardest decision in keying. Also, watch out for the effects of lighting. Daylight is best. Cool florescent light is usually fine.
Incandescent light can make lichens look yellow.
When in doubt, compare the specimen to a lichen of known color. You are looking for things like isidia, soredia, lobules, pycnidia, apothecia, cilia, whitish spots or ridges, etc. These are often patchily distributed or restricted to a small part of the thallus. Usually it will be a green alga, but not always. It's best to check until you have experience.