The air colour is caused by a physical phenomenon, the Rayleigh scattering. Absent this, the air away from the Sun would appear black, since there would be no light coming from directions others than the Sun's (basically the same that happens in space, or going high enough in the stratosphere: from the directions where no direct light source exists, we see no light, and therefore -- black. With stars).

Increasing the pressure will cause more scattering, the Sun will appear redder and the sky a stronger blue. Also, more light will be absorbed, so a darker shade of blue too (and finally black again, but without stars this time. You'd need a lot of atmosphere to do this, though; or lots and lots of suspended particulate matter, as in a nuclear winter).

The light arriving to the "far sky" has no appreciable yellow component. So, we need something to actively produce yellow light, for the sky to appear yellow.

The only way out I can see is your idea of a dispersed chemical. This would absorb the ultraviolet band, of which an A7 star has lots, re-radiating in the 580nm yellow band either through frequency halving or, much more common, yellow phosphorescence. Possibly some simple sulfur compound might do, and this could be continuously supplied by volcanism (the sky did have a yellowish tint after the Krakatau eruption, it is said).

The air colour is caused by a physical phenomenon, the Rayleigh scattering. Absent this, the air away from the Sun would appear black, since there would be no light coming from directions others than the Sun's (basically the same that happens in space, or going high enough in the stratosphere: from the directions where no direct light source exists, we see no light, and therefore -- black. With stars).

Increasing the pressure will cause more scattering, the Sun will appear redder and the sky a stronger blue. Also, more light will be absorbed, so a darker shade of blue too (and finally black again, but without stars this time. You'd need a lot of atmosphere to do this, though; or lots and lots of suspended particulate matter, as in a nuclear winter).

The light arriving to the "far sky" has no appreciable yellow component. So, we need something to actively produce yellow light, for the sky to appear yellow. Willk's answer has a very elegant solution to that.

The only way out I could see was your idea of a dispersed chemical. This would absorb the ultraviolet band, of which an A7 star has lots, re-radiating in the 580nm yellow band either through frequency halving or, much more common, yellow phosphorescence. Possibly some simple sulfur compound might do, and this could be continuously supplied by volcanism (the sky did have a yellowish tint after the Krakatau eruption, it is said).

The air colour is caused by a physical phenomenon, the Rayleigh scattering. Absent this, the air away from the Sun would appear black, since there would be no light coming from directions others than the Sun's.

Increasing the pressure will cause more scattering, the Sun will appear redder and the sky a stronger blue.

The light arriving to the "far sky" has no appreciable yellow component. So, we need something to actively produce yellow light for the sky to appear yellow.

The only way out I can see is your idea of a dispersed chemical. This would absorb the ultraviolet band, re-radiating in the 580nm yellow band. Possibly some simple sulfur compound might do, and this could be continuously supplied by volcanism (the sky did have a yellowish tint after the Krakatau eruption, it is said).

The air colour is caused by a physical phenomenon, the Rayleigh scattering. Absent this, the air away from the Sun would appear black, since there would be no light coming from directions others than the Sun's (basically the same that happens in space, or going high enough in the stratosphere: from the directions where no direct light source exists, we see no light, and therefore -- black. With stars).

Increasing the pressure will cause more scattering, the Sun will appear redder and the sky a stronger blue. Also, more light will be absorbed, so a darker shade of blue too (and finally black again, but without stars this time. You'd need a lot of atmosphere to do this, though; or lots and lots of suspended particulate matter, as in a nuclear winter).

The light arriving to the "far sky" has no appreciable yellow component. So, we need something to actively produce yellow light, for the sky to appear yellow.

The only way out I can see is your idea of a dispersed chemical. This would absorb the ultraviolet band, of which an A7 star has lots, re-radiating in the 580nm yellow band either through frequency halving or, much more common, yellow phosphorescence. Possibly some simple sulfur compound might do, and this could be continuously supplied by volcanism (the sky did have a yellowish tint after the Krakatau eruption, it is said).