From 20.6 onwards there is a dedicated button to resetting any grid colour to its default.
It is a little D at the bottom left of the recolouring menu that will automatically allow you to reset the colour.
If you have changed the default grid color in the Piano Roll or Playlist, the following values will restore it:
Hue: 135
Sat: 48
Lum: 61
Red: 52
Green: 68
Blue:78
Resetting Colors on macOS
1. Open the color picker with the '+' button:
2. Select the color sliders option at the top:
3. Select RGB sliders from the drop-down menu:
4. Enter the values noted above:
Image-Line Support Team