Secretary of State George Shultz is wearing the newest mantle of national hero with a rectitude only partly deserved, because his actual role in the Iran-contra affair did not match his masterful testimony last week.

That Shultz was badly used by Reagan administration colleagues who backed the abominable hostage-bribery scheme is clear. Even Rep. Henry Hyde, that sterling Republican conservative who serves on the Iran-contra panel, excused Shultz's witness-stand revelations about Oval Office chaos. ''He cannot defend a policy he correctly and for so long opposed,'' Hyde told us.

But Shultz's case that he did what he could to derail arms-for-hostages and save President Reagan falls short, measured against the public record. It also ignores the fact that the inner soul of Reagan's own hostage policy was the identical twin of the manipulative spymaster William J. Casey's soul. Casey, not Shultz, embodied Reagan's convictions, from arms for Iran to aid for the contras. Shultz never struggled with the ferocity needed to overcome this Reagan-Casey union.

Disputing with his colleagues was nothing new for Shultz. He had spent five years fighting rear-guard actions against Casey and other Reagan soulmates: former U.N. ambassador Jeane Kirkpatrick, Defense Secretary Caspar Weinberger (who was on Shultz's side against the Iran arms deal), Adm. John Poindexter, Lt. Col. Oliver North and others. With such experience, Shultz needed no seeing-eye dog to lead to salvation. The clear way was not to ''try to resign,'' as he says he did three times over other issues, but to resign in fact.

If the secretary had privately submitted his resignation after receiving a secret June 1986 memo from a trusted aide warning him of ''disastrous consequences'' from Iran-arms, the president might well have changed his policy. Certainly it was not beyond change, as events since November prove.

Shultz testified that before last November, when the scandal broke, not a single official in the U.S. government informed him that arms were being traded for hostages. He was challenged by nobody on the congressional investigating panel. Yet one of Shultz's most trusted advisers, Ambassador Robert Oakley, more than six months earlier sent him that ominous warning. If Shultz refused to read it, his claim to have known nothing rests on deliberate intent to learn nothing.

That can be called a ''see-no-evil'' reaction to unwise foreign policies thrust on the president over his chief diplomat's opposition.

It is scorned in the Defense Department, where Weinberger was just as much opposed as Shultz to the Iran arms sale. Reagan loyalists there seethed last week watching Shultz hang out dirty Oval Office linen. In the White House, the president, watching Shultz's testimony, ''did not react'' as he had to Ollie North, one administration source told us; that was an oblique suggestion of presidential displeasure.

Weinberger's testimony this week will serve as the flip side to Shultz's performance. It will not be susceptible to the charge of ''Reagan bashing'' that one high-level former administration official used privately against Shultz after his first day's testimony last week.

Shultz's backers, including all Democrats and many Republicans on the select committees, resent and dispute such characterizations of the secretary. In fact, they look to Shultz's performance to save their investigation after North's public relations triumph. Expressions of sympathetic understanding for the secretary by the Democrats have an additional explanation: dramatizing his heroism against administration forces of evil helps build the case they are making against the president.

Democrats praising Shultz's testimony are in many cases the same ones who have been most hostile to Reagan's policy of supporting the Nicaraguan contras. But even pro-contra Republicans, despite suspicions about Shultz's Reaganite inclinations, are loath to criticize him because they know he was so absolutely right about the Iran-arms fiasco.

Shultz's failures in the Middle East and his questionable arms-control policies are not eligible these days as targets of political attack.

But a shadow has been cast by Shultz's dissection of presidential error and caustic criticism of Casey and other comrades-in-arms. When he could not beat those colleagues, he seemed to join them with discreet, on-the-job silence. That shadow makes George Shultz an imperfect hero.