Beleiving that people of colour should be grateful for colonialism plays into even more subtle forms of racism, ones that put Western standards of living at the top of a cultural hierarchy. Many people have been conditioned to see Western ways of living (skyscrapers, financial centres, Western schools, etc) as modernity and advancement. Such misguided beliefs overlook the huge advancements made to “modern” society by people of colour and people from outside the Western world (these are too vast to list).
Such misconstrued ideas of the West’s importance are also disingenuous and overlook the historical context of the time. Britain was not as wealthy as pre-colonial India. It only consolidated its wealth as a result of the colonial project and its related thefts. India under the Mughals, for instance, was the second largest economy in the world. So even by the standards of economic prosperity as a sole detriment of progress, India was ahead of Britain.
It is now well known that the loot stolen from the Empire built modern Britain. Not only did major families consolidate their wealth during this period but major British corporations did too. There is a good case for Barclays Bank to pay reparations. Liverpool and Bristol were literally built off of the slave trade and it is commonly accepted that the industrial revolution in Britain was financed by the illegal plunder of the colonies.