Idle thought:

[cdave]

Colour is (badly) defined as the relative amount of one three frequencies of light waves reflected off a visible point.

Could you make a false colour ultrasound image by using three different frequencies of ultrasound? If so would they be of any use?

Comments

[sharikkamur.livejournal.com]

There is a fourth cone - some cones (relatively few, admittedly) are sensitive to yellow. Human tetrachromacy is a rare X-linked trait, it seems.

(Jakab Z, Wenzel K, 2004, "Detecting tetrachromacy in human subjects" Perception 33 ECVP Abstract Supplement, Wikipedia article on tetrachromacy)