I think he would lose most of his support. From what I have seen, a fairly large portion of people who are his supporters are very welcoming of minorities. They pretty much hate undocumented immigrants, communism, socialism, and - probably more than anything - “liberals”. Some of his supporters are racist, I am sure, but their views are fairly unwelcome in any mainstream groups that support Trump that I have seen.
As difficult as it can be to believe, his supporters want the unity in the country, they believe that in order for the country to be strong it must have a citizenry that is united under a few common basic beliefs. Skin color has nothing to do with beliefs, in fact, there is probably more overlap in common values and beliefs between your average Trump supporter and a random minority than a Trump supporter and a highly affluent white person from a blue are.
This isn’t to say that his supporters are not misguided, or that Trump’s record on minority issues is anything but very problematic. It is clear, if you look at what his supporters say, that they do believe Trump is the one to unify the country. Racists like Trump too, but his support goes far beyond white supremacy type racists. If Trump came out as 100% undeniably racist in the OP sort of way, he would still have some support, but only David Duke levels.