The Internet Archive has obtained some digitized American radio broadcasts from the 1940s called “The Pacific Story.”
One broadcast was about Hanoi and was entitled “Hanoi Perfume and Gunpowder.” That broadcast contains an interesting observation about “racial” relations in Hanoi in the 1940s.

To quote the broadcast:
[Narrator] “Another noteworthy phase [?] of French administration, especially to all democracy loving people, is found in the fact that the French have governed the natives of French Indochina probably with less high-handedness than any other imperial-minded people has ruled its subjects.”
[British visitor] “I say, I notice a curious thing about your French attitude in public places towards natives.”
[Frenchman] “Yes?”
[British visitor] “I mean you don’t seem to draw any color line, do you?”
[Frenchman] “Uh, ‘color line’?”
“Why, every shade of racial color is gathered right here in this café. Mongols, Tonkinese, Annamites, half-castes, quarter-castes. I must say, you treat the natives as if they were you’re equals.”
[Frenchman] “We receive them all democratically.”
[British visitor] “And, uh, do you find that a good idea?”
[Frenchman] “A very good idea, monsieur. We are after all visitors to these people’s country. We accepts one’s hospitality. Why not accept him as an equal?”
[British visitor] “But nowhere else in all the East is such a thing practiced.”
[Frenchman] “Uh, oui monsieur, I know. Yes our French clubs and French drawing rooms are open to all shades and mixtures of human beings.”
[British visitor] “What? By Jove, just look at that!”
Frenchman] “What, monsieur?”
[British visitor] “A pure-blooded French woman accompanied by a dark-skinned native!”
Frenchman] “Ah but what is wrong? Is it any more scandalous than a fair-skinned Norwegian woman being accompanied by a swarthy Spaniard to the theater in Madrid?”
[Narrator] “No one seems to know why Indochina should be the only place in Asia where such color equality should exist, or whether it has developed there as a result of government policy, or because the French people have no racial prejudices.”

I’m not sure how accurate this is, but it is interesting that someone seems to have made this observation in the 1940s.
There are many other interesting broadcasts in this series that I will discuss later. For now, here is a clip of the passage cited about, as well as a remix of this same information.
Was the interracial relations in Indochina so idyllic in the 40s ?
It stands in stark contrast with the famous report “SOS Indochine ” by the known frenchwoman jounalist Andrée Viollis who was asked by French Colonies minister Paul Reynaud to accompany him in 1932 and write an impartial report on the Indochina siutation after the uprisings of 1931 . In some pages , it reeks of “darkest Indochina ”
http://belleindochine.free.fr/Viollis-Indo%20SOS%20Indochine%20%28TEXTE%29.pdf ( the preface was written by André Malraux )
Yea, the way the author describes what I guess he saw is kind of idealistic – “racial tolerance,” and I’ve never heard that claim about French Indochina made before.
The Portuguese are the ones who are usually recognized as having intermarried with local populations the most (and that is related to their never having created a powerful colonial empire in SEA where they could enforce “racial purity.”
Still, I don’t think an American in the 1940s would call that “racial tolerance.”
So there is something strange about that report, but it’s also important to remember that after 1954 the French reportedly made it possible for “metis” to go to France, and most apparently did, so that is one form of “documentation” that has disappeared.
But still, my sense is that the Netherlands East Indies (Indonesia) is the place where there was the most intermarriage. So I don’t know why this person said what he did.