Carpet has a dark shadow

[CYUE]

I have my room newly laid a very light beige color carpet in March this year. This evening, when the sun almost set, I walked in to my room without turning on the light, I saw the carpet has a large part of dark shadow (see photo 1). I immediately turned on the light, and the dark shadow has gone (see photo 2), no trace of dark shadow at all. But when I turned off the light again, the dark shadow shows again. The carpet is very clean, I have vacuumed it thoroughly. but this still happened.

In the past months, I never notice this at all as in the evening, I always turned on the light as soon as I walk in to the room. I don't know when this happening since, and how, and why????

Any carpet expert can solve this mystery for me?

 

[CYUE]

This morning, in bright day light, that patch of carpet look yellowish from the same distance. But I examine the carpet fabric closely, they are still beige, not yellow, not dark, no dirty.