Docs: improve colour vs color consistency

API supports both, docs prefer colour

test/types/sharp.test-d.ts

@@ -713,4 +713,9 @@ sharp(input).composite([
     density: 72, 
     failOn: "truncated"
   }
-])
+])
+
+const colour: sharp.Colour = '#fff';
+const color: sharp.Color = '#fff';
+sharp({ pdfBackground: colour });
+sharp({ pdfBackground: color });