[R] need palette of topographic colors similar to topo.colors()

[bogdan romocea]

Dear useRs,

I got stuck trying to generate a palette of topographic colors that
would satisfy these two requirements:
   - the pallete must be 'anchored' at 0 (just like on a map), with
light blue/lawn green corresponding to data values close to 0 (dark
blue to light blue for negative values, green-yellow-brown for
positive values)
   - the brown must get darker for higher positive values.

topo.colors() fails both requirements and AFAICS lacks any options to
control its behavior.
  #---unsatisfactory topo.colors() behavior
  topoclr <- function(tgt)
  {
  clr <- topo.colors(length(tgt))
  clr <- clr[round(rank(tgt),0)]
  plot(tgt,pch=15,col=clr)
  }
  par(mfrow=c(2,1)) ; topoclr(-50:50) ; topoclr(-20:80)

An acceptable solution would be something like this
  grayclr <- function(tgt)
  {
  tgt <- sort(tgt) ; neg <- which(tgt < 0)
  clrneg <- gray(0:length(tgt[neg])/length(tgt[neg]))
  clrpos <- gray(length(tgt[-neg]):0/length(tgt[-neg]))
  clr <- c(clrneg,clrpos)
  plot(tgt,pch=15,col=clr)
  }
  par(mfrow=c(2,1)) ; grayclr(-50:50) ; grayclr(-20:80)
if only I could make gray() use blue/brown instead of black (I tried a
couple of things but got stuck again).

Any suggestions?

Thank you,
b.