I'll put this another way:

No Light, shadow nearby - Ok to HIPS.
Darkness - Ok to HIPS.
Light - Not ok to HIPS.
Darkness + Light - Ok to HIPS.
Continual Light + Darkness - Not ok to HIPS.
Deeper Darkness + Continual Light - Ok to HIPS.

Any other situations: If the game lets you hide. You can hide, unless the DM says no in an event.

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Dwagin [Duh-wa-GIN] N. - 1a. Small and cute thingum, particularly fond of four legged scaled creatures of the metalic and chromatic varieties.

-Extraordinarily ordinary in the mostest of ways.