I don't know as I'd describe the air as clear, exactly. If it were completely clear of dust the sky would be blue because of unimpeded Rayleigh scattering. That olive color shows up consistently in Spirit's sky pictures, and I'd say it indicates light dust-loading--a combination of blue from Rayleigh and red/orange from dust. That particular color has in fact been blessed as "real" in the "True Color of Mars" paper by Justin Maki that NASA uses as its official reference. Also, the farthest features in the picture, presumably the crater wall, are hazy and washed out--another indicator of dust.

Oppy's pictures tend to show a whiter/redder sky--far less blue component. Seems obvious that that indicates heavier dust loading.

The ground shows the opposite dust-loading. Lots of fine powder in Spirit's images, while Opportunity's images tend to show rock and blueberries that seem to have been swept clean of dust. Seems counterintuitive at first, but maybe what we're seeing are extremes. Maybe the wind is so strong at Meridiani that it sweeps the ground clear and keeps the particles suspended, while maybe at Gusev the wind is weaker and suspends less dust and allows more to settle to the ground. Or, maybe Gusev is a destination for dust deposition, and Meridiani is a source of dust.