[StructLayout(LayoutKind.Sequential)]
public struct RGBQUAD
{
    public byte rgbBlue;
    public byte rgbGreen;
    public byte rgbRed;
    public byte rgbReserved;
struct RGB {
   public TODO;
}

VB Definition:

<StructLayout(LayoutKind.Sequential)> _
Structure RGBQUAD
    Public rgbBlue As Byte
    Public rgbGreen As Byte
    Public rgbRed As Byte
    Public rgbReserved As Byte
End Structure
    <StructLayout(LayoutKind.Sequential)> _
   Private Structure RGB
    Dim byRed, byGreen, byBlue, RESERVED As Byte
    Public Sub New(ByVal colorIn As Color)
        byRed = colorIn.R
        byGreen = colorIn.G
        byBlue = colorIn.B
        RESERVED = 0
    End Sub
    Public Function ToInt32() As Int32
        Dim RGBCOLORS(3) As Byte
        RGBCOLORS(0) = byRed
        RGBCOLORS(1) = byGreen
        RGBCOLORS(2) = byBlue
        RGBCOLORS(3) = RESERVED
        Return BitConverter.ToInt32(RGBCOLORS, 0)
    End Function
    End Structure