Here, increasing intensity of green is used to represent increasing species richness of butterflies in the first map, and increasing intensity of blue is used for species richness of birds in the second map.

These green and blue maps are then overlaid in the third map. Consequently, black grid cells on the third map show low richness for both butterflies and birds; white shows high richness for both; and shades of grey show intermediate and covarying richness for both (these covarying scores lie on the diagonal of the colour key, to the left of the third map) (below):
 

 
Areas of the third map with highly saturated green cells show an excess of richness for butterflies over birds, and areas with highly saturated blue show an excess of birds over butterflies (Spearman correlation coefficient rho= 0.25). The colour classes are arranged to give even frequency distributions of richness scores along both axes (at least within the constraints imposed by tied richness scores).