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


Details:

png file /svs/share/houdini/cbars/images/png/NEO_bright_temp.png
tex file /svs/share/houdini/cbars/images/tex/NEO_bright_temp.tex
definition /svs/share/houdini/cbars/definitions/GISS_panopoly/NEO_bright_temp.act
credit
reference https://www.giss.nasa.gov/tools/panoply/colorbars/