An easier way to build custom color gradients with R.

grad.R

color.grad <- function(input, bins=10) {
  colors <- c('#FFFFFF', heat.colors(bins-1))   # white for 0/NA
  input[is.na(input)] <- 0                      # replace NA with 0
  colors[cut(input, bins, labels=F)]            # break up into bins, project colors
}