The above answer is useful but in graphs, it is difficult to distinguish between darker gradients of black. One alternative I found is to use gradients of gray colors as follows
palette(gray.colors(10, 0.9, 0.4))
plot(rep(1,10),col=1:10,pch=19,cex=3))
More info on gray scale here.
Added
When I used the code above for different colours like blue and black, the gradients were not that clear.
heat.colors()
seems more useful.
This document has more detailed information and options. pdf