A triumph for Mr. Botha

The political leaders of South Africa's 2.8 million Coloreds (people of mixed descent) have dramatically broken ranks with African opponents of apartheid in South Africa. Early this month they agreed to accept a complicated package of constitutional changes proposed last year by Prime Minister Pieter W. Botha and his ruling white National Party.

The constitutional changes will transform the present Westminster-model parliament for whites, with a prime minister responsible to parliament and a ceremonial president. It will become a tricameral parliament, with separate chambers for whites, Asians, and Coloreds, and an executive president elected effectively by the dominant party in the white parliament but otherwise largely sovereign and independent.

This arrangement, with the Colored chamber unable to legislate in combination with Asians and minority whites against the ruling party, is the one that has been accepted by the leaders of the Labor Party, the main Colored political body. That the Coloreds have done so is a major triumph for Mr. Botha.

That the Coloreds did so despite the vociferous opposition of the very Africans, including Chief Gatsha Buthelezi of KwaZulu, with whom they have long allied themselves, adds to the luster of Mr. Botha's victory. It also signifies a sharp rupture in the hitherto solid front of black political opposition to apartheid.

David Curry, the chairman of the Labor Party, urged his followers to accept the new proposals in order to permit Coloreds to influence the pace of change from within. By being a part of the process, he said, Coloreds could have influence. The Rev. Allan Hendrickse, the leader of the Labor Party, implied that by joining now, Coloreds could pave the way for a fourth chamber for Africans, and thus eventually bring the 22-million-large majority of South Africa into the political process. Both Mr. Curry and Rev. Hendrickse may also have been promised important cabinet positions in the government which will ultimately be formed as a result of the constitutional changes.

Despite their explanations, the actions of Mr. Curry and Rev. Hendrickse have already precipitated angry resignations by other leaders of the party. Youthful Coloreds are unlikely to accept their breach of black solidarity with equanimity. It is possible, too, that the protests by young Coloreds will isolate Mr. Curry and Mr. Hendrickse, limiting their effectiveness. Their predecessors in the Labor Party suffered a similar fate when they, too, chose to accept positions in, and the promises of, South Africa's white government.

Only if Mr. Botha now offers significant political concessions, or if the Coloreds win them in the new parliament, will the decisions of Mr. Curry and Mr. Hendrickse be received with favor. Mr. Hendrickse has said that ''no constitutional arrangement that does not include the largest number of South Africans can ever be regarded as final.'' But Mr. Botha has been adamant in opposing a chamber or any other form of representation for Africans in South Africa's new parliament.

In terms of the direct impact of apartheid, the Colored leaders will work for a relaxation of laws which segregate Coloreds throughout every aspect of their lives. But Mr. Botha is on record as opposing any alteration in the Group Areas Act (housing segregation) and other major pieces of legislation which both Mr. Curry and Mr. Hendrickse stridently oppose.

Now that the Labour Party has accepted the new plan, the Reform Party, the main Asian political organization, should soon follow suit. Mr. Botha will then have obtained the mandate which he seeks from the minor groups. His own party enthusiastically supports the contitutional changes. Only the Progressive Federal Party, on the left, and the Conservative Party, on the right, will oppose the constitutional alterations when they are presented to the existing parliament during its next session. But the arguments of his opponents will have been weakened by the action of the Coloreds.

Mr. Botha's transformation from a prime minister into an executive president, with wide powers, could come as early as 1984. Africans fear the use of such power. The closer the new constitution comes to enactment the more anxious they will become, and the more they may attempt to protest.