You can convert an RGB image into grayscale in Photoshop easily.
Here's a swatch of 16 colours. The values of the colours are important.
However, later versions of Photoshop add noise to the image.
The image now has 41 colours. The noise was NOT in the source image, created by hand.
If we look closer at one of the colour swatches you can see it better: I've increased the contrast - for clarification.
There are many ways to remove colour from an image. As there are ways to remove noise. That's not what I'm asking.
Using Photoshop, is it possible to convert to grayscale without the seemingly additional noise?
Image > Mode > Grayscale > No Noise, thank you.