Figure 6: The PCA results for 1022 galaxies from ALFALFA and SDSS with colors. Here we only show the strongest ones, PC1 and PC2, because other principal components are not significant by PCA criterions. PC1 is well correlated with all the variables. In the first row, the color is still correlated with the other five variables and PC1. In the second row, the rightmost plot shows that the color is even more strongly correlated with a new principal component, PC2, which is not correlated with other variables. In this plot, (g-i) and (g-i).