C# Definition:

[StructLayout(LayoutKind.Sequential)]
struct COLORREF {
   public byte R;
   public byte G;
   public byte B;
}

// Alternate
[StructLayout(LayoutKind.Sequential)]
public struct COLORREF
{
   public uint ColorDWORD;

   public COLORREF(System.Drawing.Color color)
   {
      ColorDWORD = (uint) color.R + (((uint) color.G) << 8) + (((uint) color.B) << 16);
   }

   public System.Drawing.Color GetColor()
   {
      return System.Drawing.Color.FromArgb((int) (0x000000FFU & ColorDWORD),
     (int) (0x0000FF00U & ColorDWORD) >> 8, (int) (0x00FF0000U & ColorDWORD) >> 16);
   }

   public void SetColor(System.Drawing.Color color)
   {
      ColorDWORD = (uint) color.R + (((uint) color.G) << 8) + (((uint) color.B) << 16);
   }
}

// Alternate
    [StructLayout(LayoutKind.Explicit, Size = 4)]
    public struct COLORREF {
    public COLORREF(byte r, byte g, byte b) {
        this.Value = 0;
        this.R = r;
        this.G = g;
        this.B = b;
    }

    public COLORREF(uint value) {
        this.R = 0;
        this.G = 0;
        this.B = 0;
        this.Value = value & 0x00FFFFFF;
    }

    [FieldOffset(0)]
    public byte R;
    [FieldOffset(1)]
    public byte G;
    [FieldOffset(2)]
    public byte B;

    [FieldOffset(0)]
    public uint Value;
    }

VB Definition:

Structure COLORREF
     Public R As Byte
     Public G As Byte
     Public B As Byte

     Public Overrides Function ToString() As String
     Return String.Format("({0},{1},{2})", R, G, B)
     End Function
End Structure

User-Defined Field Types:

None.

Notes:

Actually, there's no COLORREF structure in native Win32. It is typedef-ed to DWORD, which means that in the managed world its direct counterpart is System.Int32 (aka int in C#). So, when faced with interop involving COLORREF'S you'd better treat them as int's. Also have in mind that the color components are stored in reverse order, i.e. the Red component is in the lowest-byte. In short, the format is 0x00BBGGRR. You can use a code similar to the following to obtain a COLORREF:

private static int MakeCOLORREF(byte r, byte g, byte b)
{
    return  (int) (((uint)r) | ( ((uint)g) <<8 ) |  ( ((uint)b) << 16 ));
}

Notes2:

For converting COLORREF from and to .net's Color use 'ColorTranslator.ToWin32' and 'ColorTranslator.FromWin32' methods.