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

---

Details:

png file	/svs/share/houdini/cbars/images/png/blue2red.png
tex file	/svs/share/houdini/cbars/images/tex/blue2red.tex
definition	/svs/share/houdini/cbars/definitions/svs/blue2red.csv
credit	unknown
reference	unknown