Function RgbToHex(color As Long) As String
    Dim r As Long, g As Long, b As Long
    r = color And &HFF
    g = (color / 256) And &HFF
    b = (color / 65536) And &HFF
    RgbToHex = Right$(Hex$(16777216 + 65536 * r + 256 * 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. If the compiler can inline constants, 65536 can be written as (256 * 256).

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.

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.