Think; "Stop-light" red. Gauge the color of rubies from that standard. Look for consistent, uniform color throughout stone. Examine the ruby over black and white backgrounds in order to detect inconsistency in color. Something you want to avoid.
We've all heard of the "4C's" of valuing diamonds. Well with rubies, it's All about color. Now before you run out and buy the first red stone you see, we have to think about red in terms of what makes a perfect ruby. The gemstone trade considers color from three distinct, yet equally important scales.
Hue: Pure red from the spectrum of colors is where to begin. Think: traffic-signal. Now forget about ever finding a ruby that color. If you do, it's fake. But, with that color in mind, it's easier to gauge the quality of color in the real rubies you do see.
Tone: Is considered in two ways: lightness/darkness, and distribution. Basically, you're looking for hints of white, black, brown and gray. And you are looking for consistency of tone throughout the stone. If you prefer a little lighter than darker, that's fine. Keep in mind that darker stones are generally more valuable, and you want your chosen color as uniformly distributed as is reasonable.
Purity or Intensity: This can get a little dicey, so The Gemological Institute of America has prepared a handy scale for judging purity.
Vivid (now that's red)
Very Slightly Brownish of Grayish
Slightly Brownish or Grayish
Brownish or Grayish (next stone please)
Now, apply all these color signifiers to each ruby against a variety of colored backgrounds, under different light sources, from the top and from the sides. Don't forget to test the color against your skin, since there is no perfect red for a ruby, pick a red that's just right for you. Under all these conditions, keep focused on the uniformity and consistency of color. Make sure it doesn't wash out in white and disappear in black. Are the edges (girdle) the same tone as the center (culet)?