unless you find a better idea make a "doormat" out of the matching decking placed at right angles to the wall

inset door mats can look good in decks - but better say for single entrance doors to define an entry - in this case it will scream "obvious".......cheap option - replace decking with matching width with as close to original colour as possible and apply an opaque stain/colour/finish...more expensive option - replace the whole deck so you don't notice anything...........or if there's an area of the deck that you don't use much consider cutting it out for a feature garden bed, or recessing say some steps into it to savage enough matching material to fill in the gap

Thanks all - all very helpful responses - I'm getting the supplier of the decking material round to check colour match. What I failed to mention was that the house lacks a defined front door, and we generally have visitors wandering round the perimeter looking for a door, so were keen on defining an entrance. The decking is in fact a balcony with steps up from the garden. There are 3 sets of sliding doors in a row, of which this is the central one, with a cloakroom cupboard being constructed on the inside for coats/bags.

and i agree with Julie, sometimes necessity is the mother of invention and the "patterned patch" could be a unique feature of the house but does the balcony have a roof over the door? how deep and wide?