How to make a non-symmetric color pallet around zero

http://comments.gmane.org/gmane.comp.gis.gmt.user/20760

makecpt -Z -Chaxby -T-75.0/0.0/25.0 -N > paletteTwice.cpt  

makecpt -Z -Crainbow -T0.0/150.0/50.0 >> paletteTwice.cpt

psscale  -Cpalette.cpt  -D3.2/0.4/6.0/0.2/h -L -V > non_symmetric_colorbar.ps

makecpt -Z -Chaxby -T-75/0/25 -N > cpt1

​makecpt -Z -Crainbow -T0/150/50 -N > cpt2

​cat cpt1 cpt2 > cpt
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