That makes a lot of sense, and sounds like it would be the cause. But I kid you not, I can be in my room (lights on) right after I wake up, and my eyes are a really dark blue; then later in the day again in my room (again with lights on) and my eyes will be a very light blue with a dark ring.
It reflects light differently depending on the angle it enters the eye. Think mood lighting, your cornea may appear on color out doors, a different indoors under fluoriscent light, different incandescent light, difffernt in a dim cafe, etc. The iris may slowly change color over years and years but not daily.
The illumination of the iris? And how does the curvature of my cornea change?
Your iris does not change color. Any apparent change of color are due to differences in illumination of the iris and the curvature of your cornea.