Fix wxColour::GetAsString called for non-solid colour

Don't return RGB values if colour is not solid
and hence cannot be represented with these values.
Non-solid colour should be reported as an unknown
RGB value, e.g. '??????'.

Closes #18596.

interface/wx/colour.h

@@ -118,6 +118,9 @@ public:
         @c wxC2S_CSS_SYNTAX (which is the only one supporting alpha) is not
         specified in flags.
 
+        @note For non-solid (i.e. non-RGB) colour this function returns
+        "rgb(??, ?? ??)" or "#??????".
+
         @since 2.7.0
     */
     virtual wxString GetAsString(long flags = wxC2S_NAME | wxC2S_CSS_SYNTAX) const;

src/common/colourcmn.cpp

@@ -220,6 +220,8 @@ wxString wxColourBase::GetAsString(long flags) const
 {
     wxString colName;
 
+    if ( IsSolid() )
+    {
         const int alpha = Alpha();
         const bool isOpaque = alpha == wxALPHA_OPAQUE;
 
@@ -260,6 +262,18 @@ wxString wxColourBase::GetAsString(long flags) const
                     colName.Printf(wxT("#%02X%02X%02X%02X"), red, green, blue, alpha);
             }
         }
+    }
+    else
+    {
+        if ( flags & wxC2S_CSS_SYNTAX )
+        {
+            colName = wxS("rgb(??, ??, \?\?)"); // \? form to avoid ??) trigraph
+        }
+        else if ( flags & wxC2S_HTML_SYNTAX )
+        {
+            colName = wxS("#??????");
+        }
+    }
 
     // this function should alway returns a non-empty string
     wxASSERT_MSG(!colName.empty(),