How to set certain values to specific colour in R?

Question

I have one matrix r (raster). I would like to plot r , this simply can be done using

plot (r)

But I would like to mark all values of 20 (so this value will not be taken into account in the scale of legend) to red colour and plot r normally as below. a reproducible example:

library(raster)
r <- raster(nrows=10, ncols=10)
r <- setValues(r, 1:ncell(r))
plot(r)

This will produce this:

Answer 1

elegant is different but is this what you intend to do?

par(mfrow=c(2,2))
plot(r)
r.20 <- calc(r, fun=function(x){ x[x == 20] <- NA; return(x)} )
as.matrix(r.20)
plot(r.20)
r.not20 <- calc(r, fun=function(x){ x[x != 20] <- NA; return(x)} )
plot(r.not20, col="red")
plot(r.20);
par(new=TRUE)
plot(r.not20, col="red", legend=FALSE)