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


Details:

png file /svs/share/houdini/cbars/images/png/IDL_33_Blue_Red.png
tex file /svs/share/houdini/cbars/images/tex/IDL_33_Blue_Red.tex
definition /svs/share/houdini/cbars/definitions/IDL/IDL_33_Blue_Red.csv
credit none
reference https://www.nv5geospatialsoftware.com/docs/LoadingDefaultColorTables.html