The matrix is orgranized in 8 rows and 16 columns (8 red and 8 green). To address a single LED in the matrix, you have to set one of the 16 column bits and enable the coresponding row.The rows are handled by an ULN2803, an array of darlington transistors. The shift registers are used to provide column data. 16 bits are are loaded serially and put out parallely.