the various gauges are not showing red for enemies, everything is the same colour.
for example - the monitoring list isn't showing red for the Shivans.
EDIT - toggling [drop shadows] in mission had the same result.

When did this issue start? On a specific nightly or has it always been the case?

So you found this issue one 24.2.0 then and not a recent nightly? That's important clarification because there have been some extensive changes to the HUD code recently.

I happened to recently do a ton of work in the HUD code so I could immediately see why this happens.

Basically, each gauge's render method sets up everything for the render call and that includes setting the color using a special function to select the color based on gauge settings and that's where using IFF color comes in. Sometime after that is when the gauge will call the render method in hud.cpp, for example HudGauge::renderString().

In the render method is where the dropshadow is handled by first setting the color to black, rendering the string, and then here is where the bug is. It sets the color to gauge_color rather than using the original function to reset the color or some other method of saving the previously set color before flipping to black.

Should be a fairly simple fix, I think.