The easiest way to generate a palette is using generic functions from the basic grDevices package:
rainbow() topo.colors() terrain.colors() heat.colors()
These are useful if the desired number of colors doesn't exceed 7-8. The only necessary argument is the number of colors in palette.
There is also
gray() function which can be used to generate various schades of gray.
Or you could do something like:
pal <- colorRampPalette(c("red", "blue", "plum")) barplot(t(as.matrix(mydf)), beside=TRUE, col=pal(3))