How Big Is Color?

A person with 20/20 vision can distinguish between two dots that are one arc minute apart. At a distance of one arm's length, a person with normal acuity has a resolution of about 0.007". That is to say, dots that are separated by of 0.007" can just be distinguished as different dots. I would expect that the smallest identifiable color is somewhat larger than this.

For the purposes of the paper, I will assume that the smallest identifiable color is 0.025", or 40 DPI. I offer no data to support this. It is just a reasonable supposition.

On the other hand, standards for measuring color (with a densitometer or with a spectrophotometer) require that the color of a halftone cannot be accurately measured if the aperture of the instrument is smaller than 0.11".

There is an apparent disagreement here. Why is it apparently not possible (according to standards) to measure a printed halftone color at the resolution that the eye can see color? Just how big is color, anyway?