Codechange: Store Colours in Colours type. (#11625)

This reduces casts, some magic numbers, and introduces a bit of type-safety.

src/livery.h

@@ -11,6 +11,7 @@
 #define LIVERY_H
 
 #include "company_type.h"
+#include "gfx_type.h"
 
 static const byte LIT_NONE    = 0; ///< Don't show the liveries at all
 static const byte LIT_COMPANY = 1; ///< Show the liveries of your own company
@@ -76,8 +77,8 @@ DECLARE_ENUM_AS_ADDABLE(LiveryClass)
 /** Information about a particular livery. */
 struct Livery {
 	byte in_use;  ///< Bit 0 set if this livery should override the default livery first colour, Bit 1 for the second colour.
-	byte colour1; ///< First colour, for all vehicles.
-	byte colour2; ///< Second colour, for vehicles with 2CC support.
+	Colours colour1; ///< First colour, for all vehicles.
+	Colours colour2; ///< Second colour, for vehicles with 2CC support.
 };
 
 void ResetCompanyLivery(Company *c);