To answer your second question, the following line will change the comment text to a Gray color:
Set-PSReadlineOption -TokenKind Comment -ForegroundColor Gray
You probably will want to execute this when PowerShell is launched, if so, then add this to your Microsoft.PowerShell_profile.ps1 file.
And to answer your first question for a color scheme, you can add several lines changing the TokenKind value to any of the following members from TokenClassification enum: None, Comment, Keyword, String, Operator, Variable, Command, Parameter, Type, Number and Member. Also change the -ForgroundColor and -BackgroundColor values with any members of ConsoleColor.
There are color scheme projects available such as posh-monokai.