ConvertFrom-RGB.ps1

function ConvertFrom-RGB
{
    param
    (
    [Parameter(Mandatory)]
        [ValidateCount(3,3)]
        [Byte[]]
        $RGB
    )
    
    $Hexes = foreach ($Byte in $RGB)
    {
        [Convert]::ToString($Byte, 16).ToUpper()
    }

    "#{0}{1}{2}" -f $Hexes
}

function ConvertTo-Rgb
{
    param
    (
        [parameter(Mandatory)]
        [ValidatePattern("^#[A-Fa-f0-9]{6}$")]
        [string]
        $Hex
    )

    $null = $Hex -match "^#(?'Byte1'\w{2})(?'Byte2'\w{2})(?'Byte3'\w{2})$"
    $Matches.Byte1, $Matches.Byte2, $Matches.Byte3 | foreach {
        [Convert]::ToByte($_, 16)
    }
}