Although Guston mixed black with white, or red with white, he never mixed all three together, for there is no violet in these paintings. Sometimes he painted black on top of red, or red on top of black, but the three colors mushed together would have introduced a different note, that of violet. His dark red comes from red straight up, rather than from the addition of black.
There are theories as to why Guston focused on red, but maybe it was just his favorite color. In the catalogue there are reproduced two pages of a letter he wrote; it is written in red ballpoint ink on blue lined yellow legal paper, an absolutely hideous color combination. But a red ink pen? He would have had to search that out from all the easily available blues and blacks. Red probably did gather certain meanings for him, but if so, he never revealed them. There are just some things you have to keep within the safe red chambers of your heart.