[R-sig-Geo] non symetric color scale

[Marie-Anne C Serve]

I would like to create a color scale covering values from -0.032 to 0.0025 with a neutral color (say grey or white) at 0. I tried to use designer.colors but i understand that the values have to be between 0 and 1. is there any simple mean to use either this function or another to get what i am looking for?
here is what i tried:
filled.contour(1:185,1:185,O3diff[,,1],xlim=c(1,185),ylim=c(1,185),asp=1,xlab="Xcell",ylab="Ycell",
designer.colors(n=20,col=c("darkblue","white","darkred"),seq(-0.032,0,0.0025)),
zlim=c(-0.032,0.0025),key.axes=axis(4,seq(-0.032,0.0025,by=0.0005),asp=1))