When lettuce sprouts it's small and cute - the bunny of the salad greens. The cotyledons spread wide open on the soil surface. Red varieties of lettuce will change color if it's warm and sunny.
Spinach cotyledons are long and slender, forming a 'V' shape.
All brassicas have heart shaped cotyledons. This includes leafy greens and cole crops (cauliflower, cabbage etc). The picture above shows tokyo bekana, kale and scarlet frills. Cotyledon size varies between varieties, and so some brassicas make good microgreens. Broccoli, although not a salad green, is a good example.
Members of the beta family, including swiss chard and beets, are also good microgreens because their stems are often bright colors. I've taken a lot of photos of them because of this. Their cotyledons are oblong with a slightly pointy tip.