Approximately 150 young men will be trying to catch a rainbow this morning along a stretch of roadway in Rock Creek Park. The winner of the road race portion of the Junior World Cycling Championships for the next year gets to wear the rainbow-colored jersey that signifies he is the world's best young long distance bicycle racer.

The 76-mile race is the sixth of seven events that comprise the championships - the first international bicycle races held in the United States since 1912. Wednesday's four-man team trials along the George Washington Parkway in Maryland will wrap up the event. The Trexlertown (Pa.) Velodrome last week hosted five individual and team sprint championships.

Thousands of spectators are expected to line the course for the 10 a.m. race, which starts and finishes near Military Road and Beach Drive. The roads along the course will be closed to regular traffic from 8 a.m. to 4 p.m. Free parking is available in the Carter Barron parking lot near the starting line.

Thirty-three nations have entered teams in today's event, with each team consisting of up to six riders. Racers will circle the 7.6-mile route 10 times, climbing hills over much of the course. The teams will leave the starting line en mass, placed in alphabetical order according to the name of the country.

The Rock Creek course was chosen for its difficulty. With the absence of long downhills the racers cannot coast much. They are thus forced to attack the other riders throughout the race. This factor, combined with Washington's normal heat and humidity, makes the race especially energy-sapping. At three feeding stations along the route team officials will offer slices of fruit and canteens of liquids such as tea, glucose mixtures and water to streaking riders.

The cyclists will push their exotic machines along at average speeds of 25-30 miles per hour, with periodic half-mile sprints up to 40 mph. The variety of grades combined with the constant need for all-out effort, will necessitate the use of 8-10 gears rather than the normal 4-5 used during a road race.

The bicycle bear little resemblance to store-bought vehicles. Graphite and titanium frames, aluminum alloy wheel hubs and silk tires that are glued to the wheels run the cost beyond $1,000. To further lighten the 19-21-pound cycles, the racers have drilled holes in the brake levers. Racers use either 10 or 12-speed gears.

The riders themselves are built along slim, wiry lines as opposed to the broad-chested, big-muscles sprinters. The better road racer will generally be the individual with the greatest proportion of strenght-to-weight.

Last year's individual winner, Ronny van Holen of Belgium, aged past the 18-year old limit of juniors, is not here. But the Belgian team is expected to provide a stern challenge to a strong East German team. The East Germans won five of seven gold medals in last year's overall championships. Last week in Trexlertown, East Germany and another strong contigent from the Soviet Union split four of the five sprint gold medals, Belgium took the other one.

The relatively-inexperienced American team is not given much chance at placing high in the race. The best of the bunch is Greg LeMond of Reno, Nev., the reigning National Junior champion. Jeff Bradley of Davenport, Iowa, who finished second in the junior nationals, and Greg Demgen of La Crosse, Wis., are the next best American riders.

The Europeans are considered stronger because of the greater number of races in Europe. Across the Atlantic, bicycling is second in popularity only to soccer. The foreign teams also have traveling mechanics and masseurs, as well as access to more spare parts and equipment. The Americans make do mainly on their own resources and what meager sponsorship they can obtain.