Is there a way to make the marching ants red when I’m working on a grayscale image for example?
Short answer, no, it’s deeply hardcoded in the application.
However, you might find it occasionally helpful to toggle Quick Mask mode on and off (default key: Q, I believe). Quick Mask converts your selection to a virtual monochrome layer you can paint on to alter the shape of the selection area itself. Turning it on briefly can give you a bright visual indication of the exact area currently selected. You can configure the quick mask appearance (color, and whether the colored portion indicates the selected area or non-selected area) by double-clicking the button beneath the Primary/Secondary color indicator.
That’s helpful, thank you.