heatmap.2 - centering the color ramp

Hello,
a short question. How would one manage to have the color range centred
around 0 instead of assigning the extreme colors (say red and green) to
the lowest and largest value? Black should be at "0" and not at the mean
value between the largest and the smallest value. I need to compare
different heatmaps.

Thanks for any suggestion
Hartmut

Here some simple Data:
mdat <-
matrix(c(1.09761079662642,0.552868871011303,0.236339539168374,-0.1568201
09742826,0.0908534304511135,-0.500217879852688,-0.182786075741673,-0.248
107861595691,-0.362157939675895),ncol=3,nrow=3,dimnames=list(c("C1","C2"
,"C3"),c("T1","T2","T3")))

 heatmap.2(mdat,Rowv=NULL,Colv="Rowv",
scale="none",col=greenred(75),key=TRUE, symkey=FALSE,
density.info="none", trace="none",dendrogram="none")

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