Just as the Los Feliz Boulevard taillight show begins, about 5 p.m., the easygoing fountain at Riverside Drive lights up for the first time that night. Bright, rotating combinations of amber, blue, red and green bulbs illuminate the 50-foot gusher of water--water so saturated in blazing color it looks to be dyed, so unexpectedly joyous it takes restraint not to jump out of an idling Toyota and join in the dance. The 60-minute show repeats every hour until 11 p.m. The spectacle of changing lights--from hot pink to fiery red, deep purple to icy blue--plays out every six minutes. The fountain spray pattern changes every 10. Prodded by the Los Feliz Improvement Assn., the Department of Water and Power restored the fountain to its original luster and rededicated it last month. It replaced the bulbs and electrical wiring and saved the controller that activates the color and jets. This last act preserved the decades-old lights and water cycle. Constructed in 1940, the fountain rests on a spot considered holy in Los Angeles history. At 25, William Mulholland lived in a shack here, charged with studying a modest irrigation ditch. It was the first trickle under the jurisdiction of the "water giver" who would later oversee building of the L.A. aqueduct. The William Mulholland Memorial Fountain now circulates 2,250 gallons of water a minute--a geyser shooting into the sky but a trickle compared to the millions of gallons Mulholland's aqueduct pumps into the city every day.