How to remove blocky artifacts in photos in Photoshop
Watch the video for a more detailed explanation of how this color noise reduction works.
When photos are compressed too much, blocky artifacts can appear. Another common way that these artifacts can appear is when you use the HSL (Color) sliders are push in Camera Raw.
The image below highlights some of these artifacts.

Choose Filter>Camera Raw
Open the Detail panel. You will see small arrows that are hiding many options. Click the triangle to see the advanced options.
Choose Color noise reduction. Increase the slider to reduce the artifacts.

This trick doesn’t work on every image, but it works most of the time.
I hope you found this helpful.
Colin
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how do you replace a color in PS with white? Every time I chose white in the color block and then try to paint it comes out grey, not white. What am I doing wrong?
You would have to change the brightness to turn gray into white