Weren't the revels funded by Soviets and Americans though? I imagine that without the support from the superpowers, it would be harder to fight the colonists, so if the world wars or at least the Soviets are avoided, then decolonization is going to be more difficult, if one tries to use violence.
The Soviets and to a lesser extent Chinese funded most of the anti-colonialist insurgent movements (Southern Africa) with arms and training, while the U.S. and to a lesser extent British and French funded the anti-Communist insurgent movements (Central America) during the Cold War period. Economic and military aid to neighbouring countries acting as sanctuaries was also important, but this began to dry up during the late 1980s, with American and Soviet aid to Africa reaching its nadir in 1985, and having dropped precipitously after 1990, and to next to lower levels than in 1960 throughout the rest of the decade.
Modern insurgencies can be funded through the sale of minerals (blood diamonds) or drug trafficking (Afghanistan), but it this relies on a complete breakdown of state authority. I cannot imagine how SWAPO in Namibia could really survive without Soviet and Cuban aid. Even UNITA fizzled out, having to rely on diamonds mined in Northeastern Angola.
The problem I can see during the 1990s with a lasting colonialism is the American/Western dominated world where human rights became paramount. The 1990s, before the resurgence of Russia and the growth of China was a period where the neo-liberal world order dominated. The western powers had an idealistic vision of promoting democracy and human rights around the world, at times even imposing them by force. The United States of Clinton in conjunction with the United Kingdom under Blair would have been harsh critics of a colonial power, just as they would have of been of South Africa continuing apartheid. On the converse is that any colonial empire would have enjoyed a respite in fighting from the evaporation of aid from the Eastern Bloc to the insurgents and neighbouring states.
Although not a colonial power in the formal sense, it South Africa can be cited as an imperial power in the sense that it ruled over Namibia, and enjoyed a hegemonic position in Southern Africa. Even before 1990 we already saw a shift of countries in Africa which had been openly hostile to South Africa, becoming open to a modus vivendi with South Africa, as South African companies were willing to invest while overlooking human rights abuses. The western powers were becoming more critical of their former allies in countries like Zaire, once a huge recipient of American, Belgian and French aid, was now nearly bankrupt. Beginning in the late 1980s, Mobutu began allowing South African companies to invest, and even allowed SAA to resume flights and rescinded the 1963 overflight ban. The collapse of commodity prices around 1983-1985 had already hurt the third world, in particular Africa, leaving countries there dependent on great power aid during the closing years of the Cold War, by 1990 they were increasingly dependent on the IMF. This would undoubtedly give an imperial power more leverage, but also increase instability on its borders.
Some scenarios that I can imagine are for instance one where France digs in its heels in Algeria. It would have to face a hostile Libya giving aid, and would probably lead to France intervening heavily to destabilise Libya during the 1980s. If its successful, the country could collapse into anarchy in the 1990s with a vacuum being filled by radical Islam. Alternately, France makes an agreement with a strongman in Libya and begins propping up an authoritarian government (similar to Assad in Syria). Another question is what happens to France itself? To hold onto colonies requires an anti-democratic government in the Metropole, would the Metropolitan French be really willing to fight to hold onto Algeria, while suppressing the human rights of the Arab majority?
If the Spanish decide to remain in Spanish Sahara and lets say Spanish Guinea, this is somewhat easier. Spanish Guinea is small and though Rio Muni is ample terrain for an insurgency, neighbouring Gabon was firmly in the pro-Western camp and the government there would probably be more cooperative with Spain than not. Cameroon too would probably be wary of annoying Spain, as I can see the Spanish basing aircraft in Fernando Poo that would be within striking distance of Douala and Jaounde. In Spanish (Western) Sahara, Polisario had some backing of Morocco and especially Algeria. The Spanish would want to protect their phosphate mines, and probably spend funds in building a wall (similar to Morocco's). This could also make Spain a target of Islamic insurgents.
If the Portuguese decide to dig in, the insurgency in Angola is probably the most manageable as the insurgent groups there were the weakest and most divided. After 1962, were only operational in the most remote border regions of the country, with little mineral wealth. The drying up of funding from the Soviets would be a huge boost to the Portuguese, but the end of the cold war in the 1990s might make Portugal a bit of a pariah amongst Western countries. Its government would probably be seen in a similar fashion to Milosevic in Serbia and with lower oil prices, the Portuguese might just seek a neo-colonial solution, transferring power to a mix of settlers and pro-Western Africans. In Mozambique, FRELIMO was reliant on Chinese Aid and the goodwill of both Tanzania and Zambia. The collapse of world copper prices after 1975 does aid the Portuguese (and Rhodesia) as it brings instability to Kenneth Kaunda's government, and might lead to Rhodesia in particular trying to create instability in that country. In Tanzania too, the country was largely bankrupt by the late 1980s, and I can envision the Portuguese trying to stir up instability there. Additionally, after Chariman Mao's death in 1976, the Chinese government suddenly began to reduce much of its aid to Africa, this trend continued until the 2000s. FRELIMO would have to turn to the Soviets, and they might seek a change of leadership within the organisation. In Rhodesia, ZANU would have been weakened as the Soviets try to aid ZAPU instead. Portuguese Guinea was the real achilles heal, and though the situation was a stalemate by 1973, the insurgency was the costliest in terms of military casualties and spending. The pouring in of Soviet Aid to the PAIGC along with aid to the neighbouring Republic of Guinea, coupled with the jungle-terrain would make holding onto this territory possible, but very costly. Abandoning the territory, would allow the Portuguese to reduce the number of Metropolitan Troops in Africa by a third, and continue the process of shifting the burden in Angola and Mozambique to the governments of those respective "Overseas States".