Function optimization

Hi all!
I wrote the following function:

Function FillBitmap(Buffer() As Byte, SrcColor As RGB, DestColor As RGB, Width As Long, Height As Long)
Dim Red As Double, Green As Double, Blue As Double
Dim DeltaRed As Double, DeltaBlue As Double, DeltaGreen As Double
SrcColorRed = SrcColor.Red
SrcColorGreen = SrcColor.Green
SrcColorBlue = SrcColor.Blue
DestColorRed = DestColor.Red
DestColorGreen = DestColor.Green
DestColorBlue = DestColor.Blue
DeltaRed = (SrcColorRed - DestColorRed) / Height
DeltaGreen = (SrcColorGreen - DestColorGreen) / Height
DeltaBlue = (SrcColorBlue - DestColorBlue) / Height
Red = SrcColor.Red
Green = SrcColor.Green
Blue = SrcColor.Blue
For Y = 0 To Height - 1
Red = Red - DeltaRed
Green = Green - DeltaGreen
Blue = Blue - DeltaBlue
For X = 0 To Width - 1
Buffer((Y * Width + X) * 3) = Blue
Buffer((Y * Width + X) * 3 + 1) = Green
Buffer((Y * Width + X) * 3 + 2) = Red
Next X
Next Y
End Function

But it's really slow! It takes about a second (950 ms) to fill fullscreen 800x600. How can I optimize it for better performance?

VB Code:

Function FillBitmap(Buffer() As Byte, SrcColor As RGB, DestColor As RGB, ByVal Width As Long, ByVal Height As Long)
  Dim SrcColorRed As Byte, SrcColorGreen As Byte, SrcColorBlue As Byte
  Dim DestColorRed As Byte, DestColorGreen As Byte, DestColorBlue As Byte
  
  Dim Red As Double, Green As Double, Blue As Double
  Dim DeltaRed As Double, DeltaBlue As Double, DeltaGreen As Double
  
  Dim Y As Long, X As Long, lIndex As Long
  
  SrcColorRed = SrcColor.Red
  SrcColorGreen = SrcColor.Green
  SrcColorBlue = SrcColor.Blue
  
  DestColorRed = DestColor.Red
  DestColorGreen = DestColor.Green
  DestColorBlue = DestColor.Blue
  
  DeltaRed = (SrcColorRed - DestColorRed) / Height
  DeltaGreen = (SrcColorGreen - DestColorGreen) / Height
  DeltaBlue = (SrcColorBlue - DestColorBlue) / Height
  
  Red = SrcColor.Red
  Green = SrcColor.Green
  Blue = SrcColor.Blue
  
  For Y = 0& To Height - 1&
    Red = Red - DeltaRed
    Green = Green - DeltaGreen
    Blue = Blue - DeltaBlue
    For X = 0& To Width - 1&
      lIndex = (Y& * Width& + X&) * 3&
      Buffer(lIndex&) = Blue
      Buffer(lIndex& + 1&) = Green
      Buffer(lIndex& + 2&) = Red
    Next X
  Next Y
End Function