Basil Leaves Turning Red and Colourless

I planted sweet basil about 3 weeks ago (Grand Rapids, MI, USA) and several of the leaves are starting to turn reddish in hue and lose their color. Does this sound like a fungus issue or soil?

The reddening of the leaves is very likely a sign the plant is under stress. Under stress, many plants produce anthocyanins which give the leaves a reddish colour. If the stem is not turning black it is not likely the dreaded Fusarium wilt of basil and more likely an environmental problem.
Basil is very sensitive to cold and since we are having a rather cool spring in our area, You might be in the same predicament. If you planted it out three weeks ago it might be suffering from the cool nights. Protect the plants with newspapers when the night temperature is supposed to go down below 15 degrees Celsius (about 60 degrees Fahrenheit).