Here is a simple script I wrote which was inspired by this post on terminal color highlighting and by ColorDiff that does essentially the same thing.

# Out-ColorDiff.ps1
Process {
    if ($_) {
        foreach ($line in $_) {
            if ($line -match '^[<|-]') {
                Write-Host -ForegroundColor red $line
            }
            elseif ($line -match '^[>|+]') {
                Write-Host -ForegroundColor green $line
            }
            else {
                Write-Host $line
            }
        }
    }
} 

Here is a screenshot of sample output from the script:

You can use it by piping the diff output to the script, like 'svn diff somefile | out-colordiff' or if you're stuck using something like MKS, you can use 'si diff somefile | out-colordiff'.

Possible (and easy) additions would be to add direct parsing of a file instead of taking an argument off of the pipeline. This is all I need currently, but if you wish to add more features, feel free to leave them in the comments.