Q:

R custom color theme in levelplot

R
range(diffRain)
[1] -2.59  7.59

How can I code a color ramp following ways? 
red color ramp less than 0.0
blue color ramp greater than 0.0

This is a part of my code. 
myTheme <- modifyList(custom.theme(region=brewer.pal(8, 'RdBu')),
                                   list(
                                     strip.background=list(col='gray'),
                                     panel.background=list(col='gray')))

levelplot(diffRain ~ month * year,par.settings=myTheme,panel = myPanel,
        at=pretty(c(minR, maxR), n=8),
        xlab = "month\n San Jose Long-Term(1922-2012) Avg.  Monthly Rainfall in inches",
        main = "Monthly rainfall - Long-Term Avg.  Monthly Rainfall\n 1922-2012",
        sub =paste(names(MonthAvgRain. Longterm),"\n",MonthAvgRain. Longterm, sep=""))

trellis.focus("legend", side="right", clipp.off=TRUE, highlight=FALSE)
grid.text(expression(inches), 0.2, 0, hjust=0.5, vjust=1)
trellis.unfocus()

A:

You should have a look at http://colorbrewer2.org/, and diverging color schemes therein.  It can be imported into R using brewer.pal from the RColorBrewer package. 
library(RColorBrewer)
cols <- brewer.pal(11, "RdBu")

Or, for more than the default maximum of 11 colors, use colorRampPalette. 
cols <- colorRampPalette(brewer.pal(11, "RdBu"))

