I connected colour to composite node directely, and there is a noise I've found a noise in my image, though (ideally) it should be perfectly clean.
So I made an experiment. (Blender version 2.90.1) New file, I set the render size 40 x 40 and changed &quot;View Transform&quot; from Filmic to Standard.
In Compositing tab, I connected RGB to Composite directly. The colour is (R,G,B = 0.40724).

After rendering, I can see (Barely Noticeable) noise in rendered image. But every pixel's colour displayed at the bottom line, which appears when I press right mouse button,  remained the same.

Render Result
Then I saved it to PNG file and compared to original colour.

Saved noise.png, Noise pattern is changed

Can I make this noise gone?
This was rather a simple question. In the Post Processing tab, reducing Dither to 0 will solve this problem.

Hope this will help somebody.