This function returns accepts a string of three comma separated values and converts these to a Long value suitable for use in Excel.

Convert-RGBtoLong.ps1

Function Convert-RGBtoLong 
(
    [string] $RGBString = "0,160,175" #Teal
)
{

    $RGBLong = ([int]$RGBString.split(",")[2] *65536) + ([int]$RGBString.split(",")[1]*256) + [int]$RGBString.split(",")[0]
    Write-Verbose "Convert-RGBtoLong: $RGBString = $RGBLong"
    $RGBLong;
    
<#
.DESCRIPTION
    Function Convert-RGBtoLong accepts a string of three comma seperated values and converts these to a Long value suitable for use in Excel.

.PARAMETER RGBString
    String of three comma seperated values

.EXAMPLE    
    $connection =  Convert-RGBtoLong ("198,217,45")

.OUTPUTS
    long colour value

.NOTES
    NAME: Convert-RGBtoLong
    AUTHOR: Fergus Stevens
    LASTEDIT: 30 Jan 2014
        
.LINK
    https://gist.github.com/fergus/8567876
    
#Requires -Version 2.0    
#>     
}

The long colour values have also been tested in Word.