Cannot pass colors array to res.render

The documentation for render looks like:

res.render(file, [optional-options, optional-callback])

Given your brief description you are saying you get an error asking for a callback when you pass in the colors parameter. My assumption is your function looks like this:

app.get('/cards', (req, res) => {
  res.render('card', { cardFront: "What is four times four?", hint: "15<four x four<17" }, colors)
})

This is passing 3 parameters: card pug file, an object {} with properties to pass to the pug file, and a callback function. Each of the three are separated by a comma. What I believe you have done (without seeing your code) is include the colors property outside of the curly brackets properties object {} as above.

What you should do is include in the colors object inside of the properties object like this:

// Route for the Cards page
app.get('/cards', (req, res) => {
  // Use Express locals parameter object to set props
  res.render('card', { cardFront: "What is four times four?", hint: "15<four x four<17", colors })
})

Written this way there are only two parameters passed to render: the 'card' pug file, and the bracketed {} properties object. This is ok since the third (and second) parameters are optional.

Hope this helps! If not, copy in your code and we'll check it out.