I might need corrected, but I don't believe you can change the 'minor' grid color, but yes on the 'major'.
Its a Write-to property node called Grid colors, and you can have the X & Y grids as separate or Clustered together elements. You'll need to add 2 color boxes [Controls > Numeric palette] to your front panel (X & Y). The user's selection is a numeric value sent to the property node(s) you wired.
Good Luck with it, Doug