Riddle me this.

[Thomas]

My guess:

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A window

I could be any color.

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Suggesting that the object can emit or let through any wavelengths within the visible spectrum. Windows are made of transparent material by nature, allowing it to appear in any color (light emitted by objects behind it).

You can see me everyday.

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Windows are so commonplace that they are virtually unavoidable within our daily lives (buildings, cars, etc.), and can thus be seen every day.

I have been around for a very long time.

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Early documentation of windows date back to several centuries BC.

Sit down, you can probably see me now.

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By sitting down, you are more likely to be in a place surrounded by windows (e.g. sitting down on a seat inside a building) or the object becomes (more) apparent, for instance by sitting down on a train passenger seat.

[Shepard]

My guess:

SpoilerShow

A window

I could be any color.

SpoilerShow

Suggesting that the object can emit or let through any wavelengths within the visible spectrum. Windows are made of transparent material by nature, allowing it to appear in any color (light emitted by objects behind it).

You can see me everyday.

SpoilerShow

Windows are so commonplace that they are virtually unavoidable within our daily lives (buildings, cars, etc.), and can thus be seen every day.

I have been around for a very long time.

SpoilerShow

Early documentation of windows date back to several centuries BC.

Sit down, you can probably see me now.

SpoilerShow

By sitting down, you are more likely to be in a place surrounded by windows (e.g. sitting down on a seat inside a building) or the object becomes (more) apparent, for instance by sitting down on a train passenger seat.

I like the way tou think, even though this could be true, it is not the correct answer to the riddle.

[Thomas]

Ah, shoot. :P

Really wonder what it is, was thinking along the lines of a monitor/TV as well initially, but they aren't around for that long. It is that part specifically that continuously destroys any thoughts I can come up with (well that and the sitting part, although I think that's just a subtle hint as to where you would find it and not really a key "property" of the object).

I was also thinking along the lines of the sun, but since the suggestion of walls was already getting warm, that can't be it I suppose.

Edit: Hmm, thinking about it, a TV would kind of fit the riddle as well.

[Shepard]

Added a few hints to the first post.

[Dawson]

Could be the ceiling-lights.

[Jamie]

Ground or floor