I understand later Marxists both in Europe and elsewhere made contributions to Marxist thought that discussed what they saw as the global exploitation of a colonized proletariat by the western Bourgeoisie , but i cant find anything on what Marx himself thought of imperialism and some of its racial facets. Did he say anything about it or was he simply concerned with the worker in industrialized European nations?
Like a lot of different things, it varied quite a bit through his life. In particular, in his younger days he wrote many of the works in which many of the stereotypes of Marxist thought (such as that all history is reducible to class struggle, that societies all proceed through defined stages, all society derives entirely through material productive conditions, the state is merely created as an instrument of class struggle, etc) derived, while the older Marx refined, added nuance to, and at times entirely rejected these ideas. One fairly well known example is that in the Communist Manifesto (1848), in which he advances a program that looks suspiciously like the sort of thing that he would critique in Gotha Program (1871). Anyway, his view towards colonization isn't really any different. In 1853 he wrote this report on the British Rule in India in which he basically says that yes, British rule is abominable, and yes it had the effect of destroying the nascent industrial production in India before the Raj (I was actually quite surprised to find that argument from 1853!) but if by destroying the oriental despotic feudalism of pre-British India, then
whatever may have been the crimes of England she was the unconscious tool of history in bringing about that revolution.
It is a view that is, ultimately, typical for intellectuals of the time. The great utilitarian theorist of liberty JS Mill, for example, argued that the oriental barbarism of India justified its subjugation to the civilizing force of the British Empire. But unlike Mill, Marx would later give very vocal support to Indian rebels, and would later reject the linear model of social development. Unfortunately the more nuanced later thought of Marx tends to be scattered among notes and letters or in somewhat pedantic works like the "Critique of the Gotha Programme" which for a good reason is not as widely read as the Manifesto. "Workers of the world unite!" is a bit more rousing than "In short, this appendix also is distinguished by slovenly editing."
For a good short article on this, I would recommend this one.
Marx was not just concerned with the workers in industrialized European nations. His dialectical method brought him to take notice of the different modes and contradictions occurring in societies across the world. For example, even in industrialized England, Marx acknowledged the role ethnicity played in pitting working class English workers against the immigrating Irish.
For an erudite analysis of the subject, see Marx at the Margins, where Kevin Anderson argues that "Marx developed a dialectical theory of social change that was neither unilinear nor exclusively class-based. Just as his theory of social development evolved in a more multilinear direction, so his theory of revolution began over time to concentrate increasingly on the intersectionality of class with ethnicity, race, and nationalism" (244). Discussions of Indian communal forms of society, US racism and slavery, and Asiatic modes of production, among other topics in light of ethnicity, race, and nationalism, can be found there.
As for the specific of race, this piece answers your question: http://www.internationalmarxisthumanist.org/articles/abolitionists-marx-slavery-race-class-salome-lee
You might want to try asking this in /r/askphilosophy as well.