add list-colours command and edit error message in parseColour.

SmoulderingBeachBallCLI/ConsoleCommands.fs

@@ -29,3 +29,9 @@
 
             showEndOfCommandMessage
 
+        let ``list-colours`` () =
+            printfn "[INFO.] Listing available colours..."
+            for item in colourList do
+                printfn "%s" item.Key
+            showEndOfCommandMessage
+

SmoulderingBeachBallCLI/Validation.fs

@@ -1,4 +1,4 @@
-module Validation
+module internal Validation
 
     open System.Drawing
     open SmoulderingBeachBall.Domain.DomainTypes
@@ -25,7 +25,7 @@
             colourList 
             |> Map.find (colour.ToLower())
         | false ->
-            invalidArg "Colour" "The colour specifed cannot is invalid."
+            invalidArg "Colour" "The colour specifed is invalid."
             // "Please use the ListAvailableColours command for a list"?
 
     let buildBasicSpec iWidth iHeight mainColour path =