You should use dendrapply
(help document).
For instance:
# Generate data
set.seed(12345)
desc.1 <- c(rnorm(10, 0, 1), rnorm(20, 10, 4))
desc.2 <- c(rnorm(5, 20, .5), rnorm(5, 5, 1.5), rnorm(20, 10, 2))
desc.3 <- c(rnorm(10, 3, .1), rnorm(15, 6, .2), rnorm(5, 5, .3))
data <- cbind(desc.1, desc.2, desc.3)
# Create dendrogram
d <- dist(data)
hc <- as.dendrogram(hclust(d))
# Function to color branches
colbranches <- function(n, col)
{
a <- attributes(n) # Find the attributes of current node
# Color edges with requested color
attr(n, "edgePar") <- c(a$edgePar, list(col=col, lwd=2))
n # Don't forget to return the node!
}
# Color the first sub-branch of the first branch in red,
# the second sub-branch in orange and the second branch in blue
hc[[1]][[1]] = dendrapply(hc[[1]][[1]], colbranches, "red")
hc[[1]][[2]] = dendrapply(hc[[1]][[2]], colbranches, "orange")
hc[[2]] = dendrapply(hc[[2]], colbranches, "blue")
# Plot
plot(hc)
Which gives: