Which Boris Johnson is your favourite? Are you, perhaps, an admirer of today’s apparently more liberal foreign secretary, who reportedly clashed with the prime minister as he demanded a wider amnesty for Caribbean immigrants from the Windrush generation? Or are you a devotee of the Johnson who led the leave campaign to victory, thanks in large part to opposition to immigration?

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It can be hard to keep up. For just as Johnson famously wrote two Telegraph columns – one coming out for remain, the other for leave – only deciding at the last minute which to publish, so he is difficult to pin down on matters of migration and diversity. The latest incarnation has Johnson becoming “very agitated and annoyed” with Theresa May, according to the Spectator, as he pressed the PM to show generosity to migrants from the Caribbean who came here as citizens decades ago and yet have been stripped of their rights after failing to meet Kafkaesque demands for paperwork.

This is a Johnson familiar to voters in London especially, who were wooed by multicultural Boris as he campaigned to become mayor. Back then too, besides constantly reminding everyone he was one-eighth Turkish, he was fond of floating the amnesty idea – suggesting during his 2008 bid that it might be applied to the capital’s estimated 400,000 undocumented migrants. But once the mayoralty began to matter less to Johnson, and the ultimate prize of the Tory leadership and Downing Street began to matter more, we started seeing rather less of multicultural Boris. The shift was complete during the 2016 EU referendum campaign, as Johnson happily fronted a campaign that did not hesitate to stoke anti-immigrant sentiment. Suddenly the one-eighth of Johnson that is Turkish fell silent as Vote Leave issued a poster fraudulently claiming that “Turkey (population 76m) is joining the EU”, accompanied by an image of dirty footprints making their way through a door into Britain.

Both these Johnsons have a backstory. While benign Boris was the man who ran for City Hall, bigoted Boris had delighted Telegraph readers in 2002 with a column suggesting the Queen had “come to love the Commonwealth, partly because it supplies her with regular cheering crowds of flag-waving piccaninnies”. In that same column, he wrote that Tony Blair could look forward to a trip to Congo where “the tribal warriors will all break out in watermelon smiles to see the big white chief touch down in his big white British taxpayer-funded bird”. That same year, he reassured Spectator readers that colonialism in Uganda had not been all bad: “If left to their own devices, the natives would rely on nothing but the instant carbohydrate gratification of the plantain.”

Does his clash with May across the cabinet table suggest cuddly Johnson is triumphing over his nasty twin? I wouldn’t bet on it. The more likely explanation will be coldly political. As a former London politician, he will understand better than most Tories that the Windrush scandal could cause the party lasting electoral damage – starting with the black and minority ethnic voters who will not forget how a generation that devoted their lives to Britain were nevertheless treated like criminals and intruders in their own country.

Profile How the Guardian broke the Windrush story Show Hide In November last year, Paulette Wilson (left), who has lived in the UK for more than half a century, spoke to the Guardian's Amelia Gentleman about her treatment at the hands of the Home Office - and revealed that she had been held at Yarl’s Wood detention centre and threatened with deportation. It was the first of a series of stories that developed a picture of how many members of the Windrush generation were being mistreated by the government under the so-called ‘hostile environment’ policy. By February, with other examples mounting, the government had relented in Wilson’s case, but faced acute criticism from Caribbean diplomats who urged the Home Office to adopt a “more compassionate” approach. In March the story of Albert Thompson - who had lived in Britain for 44 years but was told to produce a passport or face a bill of £54,000 for cancer treatment - forced attention back to the growing crisis. After the Guardian reported a string of additional cases matters came to a head when Theresa May refused to meet with Caribbean diplomats to discuss the issue, prompted fury among opposition MPs and a wider media backlash. After days of negative publicity, the then home secretary, Amber Rudd, and May were forced to change tack and issued apologies, promised reforms – and eventually gave the Windrush generation a fast-track to citizenship. Photograph: Fabio De Paola

He will also understand – or he should – that that fury will not be confined to BAME voters. One of David Cameron’s better insights, which informed his modernisation efforts, was that plenty of white voters, especially in urban seats, look for an enlightened approach to race as evidence of a more general embrace of modernity. Cameron made advances on that score, which the Windrush scandal has surely reversed. It has retoxified the Tory brand. The first evidence could come in next week’s local elections, where Labour look set to hammer the Conservatives in London.

So if Boris Johnson is suggesting the Conservatives make nice now, it won’t be because he’s suddenly discovered his inner diversity champion. It’ll be because he sees the political cost, one that could only increase as the scandal spreads to include other Commonwealth citizens, as my colleague Amelia Gentleman reveals today. But, this being Johnson, he will also have seen the potential personal benefit in placing himself on the other side of this episode – watching as May and Amber Rudd, both of whom stand in the way of the prize he covets, squirm.

In other words, it’s futile to work out which is the true Johnson, immigration liberal or hardliner. There is only one principle to which Boris Johnson consistently holds true – and that is the advancement of Boris Johnson.

• Jonathan Freedland is a Guardian columnist