That's exactly the problem of a science called psychophysics. Regarding colors (much easier to handle than smells), it's not possible to describe the "redness" of the color Red, but it's possible to compare [u]two[/u] colors, and to judge whether they are equal or not. Color equality (or identity) is the starting point of all color science. Fascinating topic.
@helenS I think this is [u]partly[/u] a problem of language.
Ultimately, we cannot accurately describe a sense perception in any way other than with a specific name. For example, in music we can describe a note precisely with notation:
By using a different language we can communicate what could only be approximated by vocal imitation. Each note can be defined precisely as being of a specific wavelength and duration modified by the instrument that plays it, and further modified by notes such as allegro, pianissimo etc.
In the science of cooking, there are 14 identifiable fragrances which could be thought of aws being like a spectrum: fruity, citrus, floral, green, herbal, vegetable, caramel, roasted, nutty, woody, spicy, cheesy, animal, chemical. Each of these has subsets of specific aromas. All fragrances can be described as a combination of dominant and minor mixes of these and of stronger or weaker intensity. Cooks can use this to describe flavours to one another, but for them, the most accurate descriptor is the recipe; they can accurately imagine the flavour from the proportions of ingredients, preparation, cooking and presentation techniques, and then replicate it. Similarly for wine and cheese makers and other specialists like coffee or chocolate buyers. Just as some people are born with little or no smell, others are born with super-noses. The skills can be enhanced with conscious practise.
In perfumery, scents are roughly divided into top, middle and base notes. The top scents evaporate within an hour; these are the citrus aromas followed by the floral. The middle notes are herbal or spicey, lasting up to four hours. The base notes are woody, such as sandalwood, cedar or pine, and can last all day. A fixative of plant, animal or synthesized origin is usually added to make the fragrance last longer. It can have a scent of its own or none. Contrary to what one might expect, perfumes are made to appeal to the wearer, not to the opposite sex. Women's perfumes are usually attractive to women but not men, and vice versa.
Most humans can differentiate over 2 million colours. The fashion industry has devised a number code in order to communicate specific colours accurately. Artist's colours are usually named for the pigments they are derived from. Photographers, film makers, theatrical lighting designers and computer technicians communicate in terms of the spectrum of light - that is, from the colours of the rainbow and their intersections. All languages that distinguish six colors contain terms for black, white, red, green, yellow, and blue. Most languages have terms for about 11. Theo van Leeuwen is one of the master teachers of visual communication, and his new book, The Language of Colour, goes beyond the usual sources in history and psychology to propose a social semiotics of color, providing concrete examples and exercises to dazzle the eye and the mind.
But sadly, when it comes to the crunch, though we might hear a word describing a sensory perception, we cannot communicate about it unless the listener can perceive with that same sense and speak the same language. I am sorry. At least there are some compensations via language. If someone says "I smell gas (or smoke)" or "this food smells off", you know that they are communicating a specific type of danger that requires action.
