Colour in a coloured stone is three things at once: hue (which colour), tone (how light or dark), and saturation (how pure and strong). A pink can be pale and grey, or vivid and glowing, at the same carat weight, and the difference is almost entirely saturation.
The stones that hold their value glow. They carry colour to the edges and hold it under different light rather than washing out to grey. A pale, greyish pink is a pretty stone; a vivid, saturated pink is a scarce one.
Australia's Argyle mine, now closed, set the reference the trade still uses for pink diamonds, grading them by intensity from faint to vivid. When you read one of our lots, that is what we are describing: not just that it is pink, but how strongly, and how evenly.