Pay cuts, thus far only a budget nightmare for white-collar federal workers, have already become a reality in Los Angeles and parts of Colorado for several thousand low-paid blue-collar civil servants whose pay rates are linked to the private sector.

Their paychecks have already shrunk, and so may those of workers in other parts of the country where private companies are cutting salaries and whose unions are negotiating give-backs to keep firms in business.

The cuts made earlier this year are the first that federal employes have taken since the Great Depression of the 1930s.

Unlike those of white-collar federal workers, who get the same annual pay raise in January, the salaries of blue-collar federal workers are adjusted at different times of the year in 140 geographic areas. The raises -- and now the cuts -- are based on the home-town salaries paid for similar private sector jobs.

Nationwide, there are about 450,000 federal workers in blue-collar, wage-board positions, ranging from unskilled laborers to highly skilled craft workers such as carpenters and electricians. There about about 18,000 in the Washington-Baltimore region.

The federal blue-collar pay cuts in California and Colorado were made to reflect wage trends in the private sector, as measured by the government. The feds, mostly janitors and food service workers, were making hourly salaries of $6.92 to $8.29. Pay cuts they took earlier this year ranged from 4 to 44 cents an hour.

Most blue-collar workers, especially those in higher-paying jobs, got increases capped at 3 1/2 percent that reflected pay changes in the private sector.

The 5 percent pay cut that President Reagan has proposed for white-collar civil servants must be approved by Congress. But for the blue-collar employes, the cuts are a fact of life and are likely to spread to other parts of the country.

For instance, federal officials say that because of compensation cuts by Eastern Airlines, federal aircraft mechanics in Puerto Rico, whose pay rates are tied in large part to those of Eastern's mechanics, may have their salaries cut accordingly.

Wage experts for the American Federation of Government Employees say that because of pay caps, some skilled craft workers are already earning 11 percent less than private sector employes doing the same jobs in the same towns.

The next blue-collar pay adjustment for workers in the Washington area is due in October. Any increase or decrease in pay for employes will depend on the private-sector wage picture here and what Congress does about the president's pay-cut proposal.