You want to use different colors for negative and positive-valued bars.

We’ll use a subset of the climate data and create a new column called pos, which indicates whether the value is positive or negative:

library(gcookbook) # For the data set
csub <- subset(climate, Source=="Berkeley" & Year >= 1900)
csub$pos <- csub$Anomaly10y >= 0

csub

   Source Year Anomaly1y Anomaly5y Anomaly10y Unc10y
 Berkeley 1900        NA        NA     -0.171  0.108 FALSE
 Berkeley 1901        NA        NA     -0.162  0.109 FALSE
 Berkeley 1902        NA        NA     -0.177  0.108 FALSE
 ...
 Berkeley 2002        NA        NA      0.856  0.028  TRUE
 Berkeley 2003        NA        NA      0.869  0.028  TRUE
 Berkeley 2004        NA        NA      0.884  0.029  TRUE