Get real colors like white or black

I set Skybox solid color to Black but I got only grey.

When I set camera to render to texture then it seems I am getting real colors like black and white instead or lighter and darker grey tones.

I wish to understand what makes that effect.

Also I created shader that returns color only and assigned to box and still I can’t make black box. Only grey box appear on screen.

Shader "Unlit/Color"
{
Properties
	{
        _Color ("Color", Color) = (1,1,1,1)
	}
  SubShader {
        Pass {
            CGPROGRAM
           		#pragma vertex vert
			#pragma fragment frag
			
			#include "UnityCG.cginc"

			struct appdata
			{
				float4 vertex : POSITION;
			};

			struct v2f
			{
				float4 vertex : SV_POSITION;
			};

	             fixed4 _Color;
		
			v2f vert (appdata v)
			{
				v2f o;
				o.vertex = mul(UNITY_MATRIX_MVP, v.vertex);
				return o;
			}
			
			fixed4 frag (v2f i) : SV_Target
			{
				fixed4 col = _Color;
				return col;
			}
            ENDCG
        }
    }
}

I think if I assign _Color to be black then it should appear black on screen. But may be there some hidden rendering pass. Do I able to disable that pass or tune it? or at least read about it. How is it named?

It was Panel in Canvas. By default it is fills with white color and some alpha.