These two colors, in addition to black and white, make an infinite number of colors and values when mixed together. This remains true regardless of the two colors you choose. The reason I choose yellow ochre, is because it is a middle value color and this encourages the artist to lighten it or darken it so that clear value contrast can be seen. The reason I choose cadmium dark red is because it is a dark color and this encourages the artist to lighten it if a light or medium value red is desired.