It is one of the greatest weapons teachers have at their disposal for alerting pupils to their mistakes. Mightier than the sword, red ink is the established colour of choice for showing children they Could Do Better.
But according to one Tory MP, a school has banned the use of the scarlet striker – presumably because it doesn't want kids to feel bad about themselves.
Bob Blackman told Mail Online that a secondary school in his Harrow East constituency had banned teachers from using red ink.
He revealed: "A teacher contacted me and said I cannot believe I have been instructed by my head to mark children's homework in particular colours and not to use certain colours.
"It is all about not wanting to discourage youngsters if their work is marked wrong.
"It sounds to me like some petty edict which is nonsense. It is absolutely political correctness gone wild.
"My take on all this is to say children need to understand the difference between what's right and what's wrong."
Mr Blackman was so concerned about the ban that he tabled a parliamentary question asking whether the government issues guidelines which 'prohibit or discourage the use of red ink for the purposes of marking or commenting on students' schoolwork'.
Elizabeth Truss, the ministers responsible for school attendance and cutting bureaucracy, insisted: "No, the Department does not issue guidelines which prohibit or discourage the use of red ink for marking student's schoolwork."
Mr Blackman refused to name the school to protect the teacher who had spoken out. But he said he was going to take the issue up with the headteacher to ensure pupils were told when they had got things wrong.
He added: "If they have got their homework wrong they need to be told it is wrong and to understand what the right answers are. The idea that they should use this or that colour is madness."
The anonymous school's actions have been slammed by Chris McGovern, chairman of the Campaign for Real Education.
He said: "In my own experience of 35 years in teaching is that children actually prefer teachers to use red ink because they can read comments more easily.
"I think this research is misguided. The problem with using a colour like green or blue is that it's not clear. A lot of schools seem to have a culture where they don't like critcising children but actually this helps them.
"It's not intimidating children want to see where they've made a mistake. I think it's a rather silly idea."