Level 21 - it's a lot of work, but it's pretty easy to solve.

First, write down all the colours written there (don't change it to
the correct colour, leave it as it is).
For example the first one, blue will be blue, don't change it to
red.
After you list them down, take a look at the source code.
You will see this -

Do something to this and with all the colours you've listed
earlier.
What do you get?
Move on from there. The answer is very obvious...