I suppose the yellow peg is facing the green peg, in which case the green peg must be guanine.
The bases are associated in pairs. In each pair there is always a purine associated with a pyrimidine. The association is by weak bonds: the hydrogen bonds, two bonds for an AT pair; three links for a GC pair. When we know the sequence of one strand we can deduce the sequence of the other strand. The two strands are oriented opposite to the 3 'and 5' ends (the strands are called antiparallel).