What does "crayon is forever." mean?

• Pencil – inarguably erasable
• Crayon – cannot be erased

Think of the color pencil leaves – grey, so it is very dull as compared to the colors that crayons can leave. The colors here could allude to the ardour, the colorful happenings of human civilization, whereas those happenings in the past which play insignificant roles are monotone, and therefore easily forgotten.

The quote is from Emilie August's novel "The Asylum for Wayward Victorian Girls".

The reference to crayon is almost certainly made because inmates in asylums were only allowed crayon as a writing instrument (because it's very hard to hurt yourself or someone else with a crayon). In Victorian times children and young people were often committed to asylums not for any genuine mental problem but for socially undesirable or inconvenient behaviour.

It is possible that the history being referred to is people's own personal history, and "crayon" (the writing implement of the asylum) is being contrasted with "pencil" (the writing implement of the Victorian schoolchild), and meaning that your time in school may be easily forgotten, but your time in an asylum will be remembered forever. Or it may be an assertion that the history of the asylum inmates will not be easily forgotten.

The phrasing does not follow standard formal syntax of English, but it is 'the way you say it' even if it is a bit informal.

In

History written in pencil is easily erased, but crayon is forever.

the second phrase is an elision of the more detailed:

but history written in crayon is forever.

There are two interesting parts about this that are informal. One is the use of 'crayon' as a metonymy of the longer 'history written in crayon'. It's just a clever way of shortening a phrase.

As to '...is forever', that is style that your high school teacher would mark in red and your newspaper editor would fire you for, but informally sounds just fine. What those two would suggest instead is '... lasts forever'. Which is perfectly standard and formal and boring.