Here is what the Photo-Lab Index from Morgan & Morgan has to say about it:
Triethanolamine is a water soluble oil, and is in the holographer's chemical cabinet as a plumping agent to control the thickness of a silver halide gelatin layer to control the final replay color. The gelatin swells in an aqueous solution, and if triethanolamine is in it, it will remain after the water evaporates. By soaking in TEA after development and bleaching, the fringe system expands, expanding the spacing of the Bragg structure, resulting in a longer replay color wavelength, making a green hologram replay in the red, for example.
If the hologram is soaked in the TEA before exposure, the fringe spacing in the bloated state will shrink after processing, so that a blue image could be made using a red laser. Multiple soaks may be used to give multi-colored images. For details, see the Handout, Triethanolamining.
It also can also increase the sensitivity of the emulsion.
Although the PLI mentions that it is irritating to the skin, it is often used in conditioners and shaving creams.