As a matter of fact, all stars are not yellow. They appear "yellow-white" to a human eye because of the very dark-black background of the sky. And also because in the obscurity, the human eye does not use the same detectors to see than in the day light. In the day light our eyes are sensible to the colors and can distinguish between them, while in the dark / the night, we see mostly in "black and white". However, if you observe the sky at night, once your eyes are accustomed to obscurity, and in a very transparent and starry night (in the mountains, far from cities, dust and artificial lights), you will notice that some stars have a red glow (such as Arcturus), some have a blue glow (such as Vega). The color of a star is an indication of its temperature and somehow its age: Arcturus is a "Red giant", rather old star with a 'cold' (3000 to 4000 'Celsius) surface, while Vega is a very young and very hot star (more than 10000 'C at its surface).
Benoit Compte, M.S., Consultant, Paris, France
'The mathematician's patterns, like the painter's or the poets, must be beautiful; the ideas, like the colours or the words, must fit together in a harmonious way. Beauty is the first test: there is no permanent place in the world for ugly mathematics.'