I think I found this via MetaFilter, but don't remember for sure.
The piece is written for four violins (reds and oranges), two violas (yellow-green and green), and two cellos (blue and purple). Saith Wikipedia:
Felix Mendelssohn's Octet in E-flat major, Op. 20 was composed in the autumn of 1825 (completed on October 15) , when the composer was 16. He wrote it as a birthday gift for his friend and violin teacher Eduard Rietz (born October 17, 1802); it was slightly revised in 1832 before the first public performance on 30 January 1836 at the Leipzig Gewandhaus.
The person who did the visualization has over a hundred other videos on YouTube which do the same thing (sometimes in a different format), covering pieces by Beethoven, Bach, Satie, Debussy, Chopin, Vivaldi, and so if you liked this, you might also like those.