He/she may also be still be somewhat stressed, so you may not yet be seeing it's I'm feeling confident, I'm feeling good colors. The color change with mood thing is common to a lot of cichlids, not just severums, and the behavior and reasons can be subtle and complex. Going dark when hiding can be a form of camouflage-- I'm hiding in the shadows, so I'll go dark. The same fish that's light in one tank can go dark in another or vice versa, depending on tankmates, comfort level, status in a group, etc. Individuals can also vary in their reactions, with color reflecting temperament, hormone levels, breeding status... the list goes on. It can be a form of communication: I'm bad, don't mess with me, I'm interested in romance, I'm not interested, etc. and can also include reactions to what happens outside the tank-- I'm comfortable with you or used to seeing you around the tank, I recognize you as the guy who feeds me... or I don't know you and you're making me nervous. Severums can be quite interested or observant of their surroundings outside the tank ime.
There's been studies of different temperaments in fish, including extraverted vs. introverted fish, so moods and individuals can differ, or hormone levels differ for various reasons. So, as I say, it can be subtle and complex and what you pick up on with a particular fish may depend on its temperament, circumstances in your tank, your powers of observation or opportunity to observe. So what I'm saying can seem a bit anthropomorphic, until you've spent enough time watching their reactions and behavior.
Another aspect most people miss is that many fish see (and display) in ultraviolet, so we're not always even seeing the whole picture.