Function RgbToHex(color As Long) As String
        Dim r As Long, g As Long, b As Long
        r = color And &HFF
        g = (color / &H100) And &HFF
        b = (color / &H10000) And &HFF
        RgbToHex = Right$(Hex$(&H1000000 + &H10000 * r + &H100 * g + b), 6)
    End Function

Assuming the VB6 compiler really doesn't inline constant expressions, this is probably one of the fastest ways to implement RgbToHex.

Instead of using a mixture of ColorConstants and magic numbers, I have restricted my code to magic numbers only. And these numbers are not really magic, since they are all powers of 2 or very close to them.

Instead of converting every color channel separately to a hex number, it is more efficient to first combine the color channels and then convert them once.

I renamed `l` to `color` since a lowercase ell is hard to distinguish from the digit 1 or a capital i.

The number `&H1000000` makes sure that the output is always 6 digits long, by producing a 7-digit hex number first and then only keeping the last 6 hex digits of that number. If this number were not there, black would be output as `0` instead of `000000`.