Assume you are referring to a contrail rather than the jet stream which is not visible. Must admit the contrail has always appeared white to me from all angles. Sometimes from above you can see the shadow of the contrail on a cloud and this is , as you would expect, a darker colour.

Jay Trip,

Yes, it's the condensation trails (exhaust from jet engines where the water vapour condenses and forms a cloud) you are seeing rather than the jet stream as busdriver says. The jet stream is simply a flow of fast moving air from west to east so you can't actually see it. I would agree with busdriver, it is the shaddow of a contrail you seeing rather than the actual contrail. It's not always easy to see a contrail when looking down, especially if there is cloud cover below as it blends in, but the shaddow being a dark line is easily seen.

Darren

Thanks, Blame my hubby for the word jet stream!
It was looking into the sky yesterday that made me ask as there was lots of white lines from the planes and I remembered when we were flying above a plane last month to Egypt that the same lines looked black.