I also think there is a likeness with Kruse. I have attached the same pictures in another format that might be a bit clearer. According to the id document/passport, Kruse is blonde and has grey blue eyes. Although the age difference between him and Brückner is 13 years, he may well be ‘the other blonde man who accompanied Brückner and described as brothers’ (with thanks to SD, Websleuths). The new attached picture is of Kruse (unfortunately no date).
He doesn't look particularly blonde to me in his passport photo either
http://miscarriageofjustice.co/index.php?topic=9454.msg653391#msg653391 and the one of him in the boat is suggestive of a reddish tinge.
I don't think the colour of his hair in the airport photograph matters one jot. If the child is Madeleine she would have required documentation to fly so if she represented enough money to someone what is the added cost of hair dye and false documentation.
I think it is the shape of the face ~ his nose ~ forehead ~ ears ~ even the hands etc which may be the clincher plus the attitude of hiding in full sight all of which is well in Sadie's remit. Bearing in mind this photograph would never have seen the light of day but for the evacuation of of an international airport.