Last night's viewing - Twincredibles, BBC2; This Is Jinsy, Sky Atlantic

Amanda Blue's film Twincredibles had a terrible title but a fascinating subject – mixed-race families in which one child was ostensibly white and one apparently black. Neither description actually made sense, but in a society often over-anxious to categorise racial difference two children who should have been regarded as effectively interchangeable found themselves on the opposite sides of a ridge-line of pigmentation. And even for their mothers that made a difference. Sometimes it was in ways that were dismally predictable. "Someone has said that I should let Layton be brought up properly by a white family," said the mother of two small boys, one pale brown and one pinky white. But at other times the prejudices of the outside world seemed to have been unconsciously assimilated by those who had no time for prejudice at all: "With Leo, I'd give him a lot more guidance," said Shirley, the mother of two baby twins, "I don't want Leo to have this stupid bad boy image and be like totally ghetto and stuff." Leo was just a few shades darker than his baby sister, Hope, but already, it seemed, his destiny had trouble in it. And Shirley, his mother, wasn't "white" herself. She was known as "Black Shirley" on the Yorkshire estate where she lived.

The absurdity of these demarcations was highlighted by the historic experience of another pair of twins, Daniel and James, now teenagers. At nursery school, Daniel – who is much paler than his brother, James – had been obliged to describe himself as black and (incredibly) to colour himself in as black in childish self-portraits, until his mother stepped in and put a stop to it. Years later, he was still uncertain about himself and still enduring more racial abuse than his darker twin: "You're white with a black brother," explained James, "which makes you 'dirty white' to them." And although Daniel's family was absolutely lovely – a rebuke to the notion that skin colour mattered at all – one of his mother's remarks suggested that even for the least racist of us skin colour doesn't go unnoticed: "For me, it's like, 'Yay... there's someone in the house that matches me,'" said Daniel's white mother.

The documentary was full of such moments – little clues to the fact that race (and mixed-race) is far more complicated than we might wish it to be. And it constantly confounded your expectations. The sibling rivalry between Kaydon and Layton seemed to conform to a baleful prediction of racial exclusion. "We're the white team because we're white," said Layton, ganging up with another friend against his brother (something that apparently happened often enough to make his mother worry that one day he might fall in with other white boys who would be racist). But then you encountered Moesha and Ebony, two Glasgow girls, and watched Moesha pleading with her mother to slap on the fake tan and make her darker. "I think when you're black you look better," she said, a little girl visibly uncomfortable in her own skin. She and her sister were living proof that the cosmetic differences were biologically negligible and yet living proof too that they still had a profound impact on the sense of self. Blue's film, full of grace notes and emotion, was a lovely rebuke to the most familiar prejudices surrounding race, but it forced you to think a bit harder about some cherished liberal prejudices too.

If you were to try and piece together the genetic make-up of This Is Jinsy, Sky Atlantic's comedy, the family tree would run something like this: Grandfather: Spike Milligan; Parents: The League of Gentleman and Vic and Bob; Distant Cousin: The Mighty Boosh. There's a fine running gag about a talent show judged by a dog called Sandy (his paw hovers between two buttons marked "Woof" and "Enoof" after he's watched the acts). I couldn't put it better than one of the characters in last night's episode: "I think I can safely say, without fear of exaggeration, that I quite enjoyed it."