PowerShell String Formatting: Why is the colon character causing my variable’s value to be blank?

The colon is a valid character for variable names, e.g. in $Env:PATH, etc.

You can use the following option, too

$server`: $status

or, for some cases a format string is more readable:

'{0}: {1}' -f $server, $status

Back to the colon. There is a special case for variable names that correspond to an item on a PSDrive:

$Env:Foo           # equivalent to the contents of Env:\Foo
$Function:C:       # equivalent to the contents of Function:\C:
${C:\autoexec.bat} # ... you get the picture

The syntax ${} exists to be able to specify variable names that otherwise use characters reserved for other parts of the syntax. You could see it as being similar (but more powerful) to C#’s @ in front of identifiers. See above where a \ is used in the variable name, since $Drive:Item only works for the current container on a drive (or the root for non-hierarchic ones like Env, Alias or Function).

Another example where the variable name would be normally invalid syntax:

PS> $+
The term '$+' is not recognized as the name of a cmdlet, function, script file, or operable program. Check the
spelling of the name, or if a path was included, verify that the path is correct and try again.
At line:1 char:1
+ $+
+ ~~
    + CategoryInfo          : ObjectNotFound: ($+:String) [], CommandNotFoundException
    + FullyQualifiedErrorId : CommandNotFoundException

PS> ${+} = 5
PS> ${+}
5
PS> Get-Variable +

Name                           Value
----                           -----
+                              5