I'm trying to create a scatterplot matrix and i got an error

I used this -->

scatterplot.matrix(~mfgfirms+popn+precip+raindays+ temp+wind | SO2, reg.line=FALSE,smooth=FALSE, span=0.5, diagonal = 'density', by.groups=TRUE, data=USAir, row1attop=FALSE)

And I got this error:

Error: number of groups exceeds number of available colors