La respuesta que te da Edu es correcta. Sin embargo debes tener en cuenta cambiar la propiedad ScaleMode del Picture a 3 (Pixel), de Lo contrario GetPixel no funcionara correctamente.
Aqui te mando el ejemplo
Option Explicit
Private Declare Function GetPixel Lib "gdi32" _
(ByVal hdc As Long, ByVal X As Long, ByVal Y As _
Long) As Long
Dim BytRed As Byte, BytBlue As Byte, BytGreen As Byte
Dim LonColorCode As Long
Private Sub Picture1_MouseMove(Button As Integer, Shift As Integer, X As Single, Y As Single)
On Error Resume Next
LonColorCode = GetPixel(Picture1.hDc, X, Y)
BytRed = ExtractR(LonColorCode)
BytGreen = ExtractG(LonColorCode)
BytBlue = ExtractB(LonColorCode)
Picture1.ToolTipText = BytRed & " " & BytGreen & " " & BytBlue
End Sub
Private Function ExtractR(ByVal LonCurrentColor As Long) As Byte
On Error Resume Next
ExtractR = LonCurrentColor Mod 256
End Function
Private Function ExtractG(ByVal LonCurrentColor As Long) As Byte
On Error Resume Next
ExtractG = (LonCurrentColor \ 256) And 255
End Function
Private Function ExtractB(ByVal LonCurrentColor As Long) As Byte
On Error Resume Next
ExtractB = (LonCurrentColor \ 65536) And 255
End Function