hdr.den plot colors help

Dear All,

I have the following code:

a <-rnorm(5000,10,2)
hdr.den(a,prob = c(25, 50, 75,95))

is there a way to change the colors of the horizontal bars representing the given highest density regions from the current white, green, red and black to colors of the shades of grey going from lighter to darker?

appreciate the help,

thanks,

Andras