The decision last week by the colored Labor Party to accept the token role in government offered it by Prime Minister P. W. Botha is a watershed in South African affairs.

It means Botha is beginning to succeed in his strategy, dressed up as reform, of trying to co-opt the colored and Indian minorities as allies of the whites, while shutting out the African majority more rigorously than ever.

This will create an enlarged "insider" group, 7,750,000 strong instead of the present 4,500,000 whites, under a carefully worked out constitutional formula that will still enable the Afrikaner National Party to keep control in its hands.

In the process, the already badly divided black groups are being even further divided, making the continuation of white domination easier.

Not only are the Africans bitterly resentful of the decision, but the colored community and the Labor Party itself are splitting over it. The rising young colored church leader, Allan Boesak, has described it as "disgusting," and four key Labor members have already resigned. More will follow.

The decision is particularly damaging to Chief Gatsha Buthelezi, the most important black leader opposing apartheid from within the country. But Buthelezi, leader of the 6 million Zulus, South Africa's largest tribe, made an impassioned appeal to the Labor Congress here last Monday not to accept Botha's offer.

He suffered the humiliation of being ignored on his own home ground, and the Black Alliance, which he had formed to broaden his power base, has now been virtually destroyed because the Labor Party was a key part of it.

All this the Labor Party did in the hope that participation would give it a toehold in the white man's power system, which it can then use to negotiate for further concessions on behalf of Africans as well as themselves.

Or at least that is how they put it. More cynical observers feel they could not resist the perks being offered, including the possibility of a couple of Cabinet seats, and the notion that they are being accepted at last by the whites, which is something the mixed- race coloreds have always yearned for.

Buthelezi scoffs at the idea that the coloreds may be able to use their position to fight for the inclusion of Africans. He notes that Botha has pledged never to include the Africans, and says if the coloreds could not bargain for this before accepting his offer they are unlikely to succeed in doing so after acceptance.

Botha desperately needed Labor's agreement to participate in the new constitution to give it a semblance of legitimacy. It is the major party among the 2.5 million coloreds and, though moderate, has a good track record of standing up to the government against apartheid.

Had Labor said no, Botha would have been left with the same old discredited collaborators who have been used to window-dress apartheid institutions in the past. Having split his party and the whole Afrikaner volk for the sake of the new constitution, Botha needed to do better than that.

As it is, by saying yes, Labor not only rescued Botha but handed him a major success that will strengthen him.

This may help him stave off the growing threat on the Afrikaner far right. But it could also mean an erosion of support for the integrationist Progressive Federal Party, as many whites see Botha as a reformist with the courage to run risks.

This points to the most serious implication of all: the already vast gulf between the way whites and blacks see what is happening in South Africa is going to widen further.

While whites gain the impression real reform is under way at last, with credible non- white faces in parliament and the Cabinet, Africans will feel more alienated than ever and see those self-same nonwhite faces--already discredited in their eyes--as symbols of their betrayal.

To a man, Africans see this as a ganging- up of whites, coloreds and Indians against themselves.

Prince Gideon Zulu, a key figure in chief Buthelezi's Inkatha Movement, expressed the feelings of many when he said: "If that is their (the coloreds') decision, they will have to live with it. They know blacks are going to rule this country one day, and when that day comes we shall not forget."