Colors: Purple and Gold
There is some discrepancy in the origin of Royal Purple and Old Gold as LSU's official colors.
It is believed that those colors were worn for the first time by an LSU team in the spring of 1893 when the LSU baseball squad beat Tulane in the first intercollegiate contest played in any sport by Louisiana State University. Team captain E.B. Young reportedly hand-picked those colors for the LSU squad.
Later that year, the first football game was played. On November 25, 1893, football coach/chemistry professor Dr. Charles Coates and some of his players went into town to purchase ribbon to adorn their gray jerseys as they prepared to play the first LSU gridiron game.
Stores were stocking ribbons in the colors of Mardi Gras -- purple, gold and green. -- for the coming Carnival season. However, none of the green had yet arrived at Reymond's Store at the corner of Third and Main streets. Coates and quarterback Ruffin Pleasant bought up all of the purple and gold stock and made it into rosettes and badges. LINK
I've heard that the original ribbon colors were more yellow than gold. We all know that Mardi Gras gear comes in both the old "real" gold and the yellow gold. I'm thinking those first ribbons were more yellow than gold, so we stuck with them and that is where we are today.
I've also seen posts concerning the changing of the yellow's hue through the years, so it is definitely debatable.