colors in AQP plot SoilProfileCollection

Hi All
I would like to know how can I change the colors in 'plot' a
SoilProfileCollection .
The colors automatically range from blue to red.
I would like to represent the depth variation in gray scale or another
color scale.
An example of command line is:
plot(CX, name='Horizons', color='pHH20')

thanks in advance

Waldir de Carvalho
Researcher Embrapa Soils - Brazil