50 tons de cinza:

require(ggplot2)
grey50 <- data.frame(
x = rep(1:10, 5),
y = rep(1:5, each=10),
c = unlist(lapply(seq(10,255,5), FUN=function(x) { rgb(x,x,x, max=255) })),
t = unlist(lapply(seq(10,255,5), FUN=function(x) { ifelse(x > 255/2, 'black', 'white') }))
)
ggplot(grey50, aes(x=x, y=y, fill=c, label=c, color=t)) + 
geom_tile() + geom_text(size=4) +
scale_fill_identity() + scale_color_identity() + ylab(NULL) + xlab(NULL) + 
theme(axis.ticks=element_blank(), axis.text=element_blank())