Here is a really simple example
volcanoR <- raster(volcano)
#making colors below 100 red and above 180 blue in this example
breakpoints <- c(94,100,120,140,160,180,195)
colors <- c("red","white","white","white","white","blue")
You just need to pass the plot a vector of break points and a vector of colors to match the breakpoints. Check out the
RColorbrewer package for some very nice built in color ramps. Also check out the
classInt package for making the breakpoints.