For the main thing, keeping colonization to a minimum, you need to keep non-european military, along with industrial, chemical, and medical technology, from being left behind by Europe. Particularly naval technology which is honestly one of the biggest advantages Europe/The West had over everyone even to today.
Now that doesn't stop everything, the British didn't conquer India with better technology for the most part, but mainly just acting like any other local power conquering the region and co-opting the local rulers and power brokers, no different than the Mughals or Vijayanagara before them.
As for effects? Honestly depends on how you stopped it and if the locals don't do their own empire building. Southeast Asia could easily have been united under Thailand, Burma, or Vietnam, or split between two or all three, if they weren't conquered by the Chinese empire. India could end up largely united or fragmented into a dozen or more kingdoms, same with Indonesia. Same with Africa, several dozen nations, or a few powerful local empires.