add test which checkes the colour of solid colour brush.

The colour of the brush is set using individual RGBA values.

TestCentre/LibraryTests.fs

@@ -99,12 +99,14 @@
             (File.Exists sPath) = true
 
         [<Property>]
-        // need to write a test which tests the colour is the same once coverted.
-        let ``Can make a solid brush from RGBA values`` () =
-            let brush =
-                makeSolidBrushFromRGBA (newRGBANum ()) (newRGBANum ()) (newRGBANum ()) (newRGBANum ())
-            let b = brush :? SolidBrush
-            b = true
+        let ``SolidBrush colour matches the individual RGBA values`` () =
+            let a = newRGBANum ()
+            let r = newRGBANum ()
+            let g = newRGBANum ()
+            let b = newRGBANum ()
+            let referenceColour = Color.FromArgb (a, r, g, b)
+            let brush = makeSolidBrushFromRGBA r g b a
+            brush.Color = referenceColour
 
 
         [<Property>]