I have 5 posts on my porch, 90mm, that I've clad with 138mm merbau decking boards, with edges mitred, then sanded after fixing to the posts so no joins are visible. I screw fixed them to the 90mm posts, then bought some merbau colour wood filler from bunnings to fill the screw holes. It looked ok when it was going on but now it's dry its far too light!

When i sand it back it also leaves light coloured sections in the merbau which looks really bad.

I asked at the paint desk at Bunnings and they recommended staining the sections with a small brush, but this doesn't help as it colours the merbau around it too.
Any ideas would be much appreciated, I really don't want to start from scratch.
The filler I used:
