Why does an object change color depending on where I look

I made a scene with a Player which can move and look around, and a big dark blue rectangle as the floor.
However, when I look forward the rectangle looks gray/light blue.

However, when I look straight down it is the color its supposed to be(dark blue).

I also put walls around before with the same color and the same thing happened, they were gray.

Assuming you haven’t done anything else yourself, the floor is using the Standard shader, built on the principles of “Physically-Based Rendering” (often referred to as PBR). A common attribute of “realistic” materials is that they reflect different amounts of light when viewed at a steep angle.

In this case, you’re likely seeing the color of the skybox (light blue) being blended into your ground, further mixed in with a greater brightness attributed to “Rim Lighting” (likely white), also provided by the same shader.