Manipulate a color palette for "heat map" type colour schemes.

color palette.R

library(RColorBrewer)

#Assume your data are in a data frame.
df1 <- data.frame(x = 1:100)

#Define the basic palette (this case goes from red - yellow - green, but you could make it any 3 colours.)
colCode <- colorRampPalette(c("red", "yellow", "green"))(n = 999)

#Make a vector (of length 999), but break apart the parts of the sequence that should be red yellow and green.
#This allows you to alter where the yellow "pivot point" is.
numValue = c(seq(1,30,length=333), # for red
               seq(30,50,length=333), # for yellow
               seq(50,100,length=333)) # for green

#put these in a data frame
my_palette<-data.frame(colCode,numValue)

#A function to pick the closest matching numeric value from the palette.
findcol = function(x){my_palette$colCode[which.min(abs(my_palette$numValue - x))]}

#Apply this function to the data vector.
df1$colVal = as.character(unlist(lapply(df1$x,findcol)))


#Plot it.
plot(df1$x,pch=16,col=df1$colVal,cex=2)

#Now try altering lines 11-13 to see how it alters the sequence of colours.