Is gray a color?
It depends if you want to be annoyingly pedantic or not. Some people might want to refer to it exclusively as a ‘shade’ or ‘tint’.
In more everyday language (especially for artistic purposes) everything is a colour. Black, white, grey, purple. There is no confusion.
Technically I think grey is achromatic colour. As is white and black.
In physics, Black is not considered a colour, but white is (unless someone wants to correct me there).
Short answer: Yes.
Color is not a physical quantity, unless you’re referring to the property of quarks by that name (which is almost entirely unrelated). Really, color has a lot more to do with physiology than with physics. When physicists refer to color, we just mean the same nontechnical definition as everyone else.
Gray is one of the eleven basic colors words in English. See Cecil’s column Could early man only see three colors? for more this.
There’s different ways of categorizing color - CMYK, RGB, etc. Probably the most ‘common sense’ way is HSV - hue / saturation / value. Hue is basically which part of the rainbow, saturation is how colorful vs how gray, and value is how dark or light it is. The confusion comes in that the word color, while usually broadly defined to mean any HSV, is sometimes also used more specifically to refer only to hue. Under the HSV model, gray is better said to be a class of colors than a specific color, as it can refer to any hue and many values of extremely low saturation.
There are all different shades of gray, plus warm grays and cool grays. They are all colors. And if you have something that’s, say, red and gray striped, you think of it as having two colors, rather than being solid.
No. Grey is a colour.
To expand on that, what we mean when we talk colloquially about “seen” color is not just the frequency distribution of light but also the intensity and combination of different hues, which is not a linear relationship. What we call grey, for instance, is actually a frequency distribution that is approximately “white” (or, at least, in which no frequency tends to stand out over others) but has overall low intensity.
If you want to talk technically about color as in photometry, the fact is that all non-monochromatic colors are actually some mathematical superposition of three primary colors to create a color space. However, there are no particular colors that are the primary colors; any combination of three or more sufficiently separated colors or transformations thereof will work. As a practical matter, additive projected colors are usually red, blue (or violet), and green, and usually based on or close to the CIE 1931 XYZ color space, although for technical reasons some obsolete filmstrip processes and color CRT illumination used a different set of primaries, offering a more limited but presumably adequate color space.
Richard Feynman offers a particularly accessible explanation of photometry and color vision in *Feynman Lectures on Physics*, Vol I, Chapters 35 (Color Vision) and 36 (Mechanisms Of Seeing), including the specific physiochemistry of vision and why certain hues are preferred for primary colors.
Stranger
In my line of work it is not. I make dyes for sunglasses. Grey or brown lenses require at least three colors per lens, and sometimes we use as many as six.