Blue.

Is a color an object?  I think like most people I’d say it’s an attribute of an object, not an object itself.Then I stumbled on this story about the invention of a new blue (YInMn Blue).  If you can invent it maybe it’s a thing.  Or rather, maybe blue has an element of object-ness, just as it is also a perceptive trick and a mood.

When we’re sad, we’re blue.  The color does seem appropriately melancholy, evoking the infinite depths of the ocean and the cold inhospitable quality of ice.  The sky however is a bright joyous blue!  In fact blue comes in a zillion varieties (see photo of all the blues I own - if you can own them they must be objects?)

all the blues I own.

I once heard a story I've never been able to find since, about our perception of color. Apparently if you ask people to picture the exact color red of a can of con we will all actually picture a slightly different shade. On further reflection I have no idea how you would prove this, but it makes sense anyway.

At the Exploratorium there's an exhibit where you can choose the color from a surrounding ring of colors that best matches the color in the middle. It's confounding to stand next to someone and have them choose differently. We all see the world slightly differently. It's so true. 

Disagreeing about Color at the Exploratorium (click the image for more info)