If you're going to own your own biplane, you may as well make it a yellow one. There's just something right and proper about flying around in a yellow biplane.
It belongs to Denys, of course, and it's something he's apparently coveted for a long time. It's not hard to see why. The plane gives him a sense of freedom that defines his bliss: a way of getting away from it all just by firing up the motor. There weren't a lot of planes back then and you really didn't need a pilot's license—just a runway and a book of instructions.
Of course, that means that the biplane isn't entirely safe. It may explain why he eventually crashes and gives Karen an even bigger reason to head back to Denmark. The freedom he demands can be dangerous, and though he thinks he controls it, it can turn around and bite him when he least suspects it.
Somehow, we think Denys would be okay going out like he did—chasing his bliss, rather than dying in bed.