What's the easiest way to remove this line in photoshop?

Filter>Distort>Lens Correction>Chromatic Aberration>Red-Cyan fringe slider. Then from panel menu at top right of Lens Correction dialog, choose Save Settings. Open another image and choose Load Settings from same menu.

I have quite some experience with Photoshop, but it's difficult to say how to get rid of something if one doesn't know how it came about in the first place. Can you describe the "replacing the background layer (upper grey) with a mask" more precisely - ideally step-by-step - how did you select the "upper grey", and what did you do next?

And if you want the background grey, you can just recolor it grey by right clicking the paintbrush tool and having black selected and going over the picture. That will make it grey without the use of masks and you won't have the line.

I think the simplest way would be to select the line using Select>Color Range... as there doesn't seem to be anything similarly colored in the image. Then you can use Selective Color, or Hue/Saturation, or Black and White (under the menu reached by clicking the black-and-white cookie-like symbol under the layers menu) to adjust the color until it reaches the gray of the background. This is a quick way, and if you wanted even better results, you could also do a bit of touching up with clone stamp and the healing brush, but it'll remove the line quickly.

You need to see if the line can be affected by photoshop tools. You might try a magic erasing it (alt click the blue line, then click the eraser to erase the blue. Or selecting the line in a box, then adjusting the color scale.

Make it black and white to hide the color. Or use the burn tool to darken the line.

I'd really need to see a better copy of the image. I think you linked the thumbnail instead of the actual image and I don't know how to walk backwards to get the actual image. I agree with what has been said here so far I don't think there is an "easy" solution to this.

I would probably use the magic wand to select the odd color and then mess with the colors to simply make the line disappear/blend with the background. That's the only easy solution but the result will probably be less than stellar. The best way would be to get an artist to paint it out and make a guess as to what should be behind the line.

Good Luck!

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