colorRampPalette
could be your friend here:
colfunc <- colorRampPalette(c("black", "white"))
colfunc(10)
# [1] "#000000" "#1C1C1C" "#383838" "#555555" "#717171" "#8D8D8D" "#AAAAAA"
# [8] "#C6C6C6" "#E2E2E2" "#FFFFFF"
And just to show it works:
plot(rep(1,10),col=colfunc(10),pch=19,cex=3)