In the Houdini colorbar HDA under "color options", set the menus like this...

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Details:

png file	/svs/share/houdini/cbars/images/png/idl33_reversed.png
tex file	/svs/share/houdini/cbars/images/tex/idl33_reversed.tex
definition	/svs/share/houdini/cbars/definitions/svs/idl33_reversed.csv
credit	unknown
reference	unknown