Here's the breakdown on the Foldit view of DNA.
There are four DNA "bases": Adenine, Cytosine, Guanine, and Thymine, usually known as ACGT. They form "base pairs" that span the famous double helix. Adenine can pair with Thymine, and Cytosine can pair with Guanine.
Foldit gives the pairs a color and a size. So the small yellow DNA base pairs with the large yellow DNA base. The small purple pairs with the large purple. The table below shows the base name, the one-letter code, and the two-letter code returned by structure.GetAminoAcid, along with the color and size.