Nothing important but from the description of colorbar -D:

"Append +w followed by the length and width of the color bar. If width is not specified then it is set to 4% of the given length. Give a negative length to reverse the scale bar. "

So, if I want to have a reverse colorbar I must give a length. In other words, there isn't an option to use the default length and to have a reverse colobar.