Walk into any library during exams and you will see the two ways colour coding goes wrong. One page is a rainbow: six colours, a legend on a sticky note, and no way to tell at a glance what matters. The other is entirely yellow, which is to say nothing is highlighted because everything is. A colour code is a filing system for attention, and like any filing system it works only if it is small enough to apply without thinking.
The three colours and their questions
| Colour | Question it answers | Highlight this | Never this |
|---|---|---|---|
| Yellow | What is the point? | The claim, thesis or conclusion of a section: the sentence you would quote if asked what it says | Background, examples, transitions |
| Green | What backs it up? | Evidence: a number, a result, a named study, a quotation, a worked example | The author's opinion about the evidence |
| Pink (or orange) | What do I need to look up or remember exactly? | Terms, definitions, names, dates, formulas | Whole sentences; mark the term only |
Yellow carries the argument. If you could keep only the yellow lines from a chapter, they should read as a rough summary. Green is the proof for a yellow line and should sit near one; a green highlight with no yellow nearby usually means you highlighted a fact you found interesting rather than one that supports anything, which is fine occasionally and a warning sign in bulk. Pink is for retrieval: the things you will be tested on verbatim or will need to define. Pink marks are short, often a single word.
Why three and not five
Every colour you add is a decision you make on every sentence. With three, the decision is fast: is this the point, the proof or a term? With five or six, you stop to classify, and classifying feels like studying while doing nothing for memory. Research on highlighting (summarised in our review of whether highlighting works) is blunt about this: the marking itself does little; the benefit comes from what you do with the marks afterwards. Three colours keep the marking cheap and make the afterwards possible.
If you need a fourth colour, make it a temporary one: blue for "I do not understand this yet". Blue marks are meant to disappear. When you have understood the passage, they get replaced by the right colour or erased (on screen) or left as a record of what was hard (on paper).
Applying it on paper, in PDFs and on the web
- Paper: three physical highlighters. Read a section to the end before you mark anything, then go back and mark. This single habit prevents the everything-is-yellow page.
- PDFs: every viewer has multiple highlight colours (see highlighting in a PDF). Set the three as favourites. Attach a one-line note to any green highlight that does not sit next to its yellow claim.
- Web pages: extensions such as Weava and Hypothesis offer several colours; assign the three and ignore the rest. For pages you only need to point at once, a highlight link is enough and needs no colour.
- E-books: most reading apps offer three or four colours. Map them the same way so your exported highlights keep their meaning.
How much to highlight
A useful ceiling is one yellow line per paragraph and often none. A page that ends up more than about a tenth highlighted has stopped being selective, and selectivity is the only thing that makes the marks useful later. If a paragraph seems to need three yellow lines, it usually needs one yellow line and a note in the margin in your own words, which is a better study act anyway.
Reviewing with the code
The code pays off at review time. First pass: read only the yellow lines of a chapter and try to explain, out loud or on paper, how they connect. Second pass: cover the green lines and try to recall the evidence for each yellow claim; then check. Third pass: turn every pink mark into a flashcard or a two-column list. Each pass is a retrieval exercise, which is what makes the highlights earn their keep. Our guide on turning highlights into notes walks through the same three passes in detail.
Colour-blind readers and printing
About one in twelve men and one in two hundred women have some colour vision deficiency, most often affecting red and green. Yellow, green and pink can be hard to separate for them. Two fixes: choose yellow, blue and orange instead, which stay distinct for most people; or add a shape, using underline for evidence and a circled word for terms on paper, and the underline or strikethrough tools that PDF viewers offer alongside highlight. The shape rule also survives black-and-white printing, where every colour becomes grey.
The same three-colour idea adapts to other jobs in the study section: marking writing habits during close reading uses a small set of marks for patterns, and classroom highlighting activities show how to teach the code to a class.
Common questions
What do highlighter colours mean when studying?
Whatever you decide, consistently. A workable default: yellow for the main point, green for evidence, pink or orange for terms and definitions. The meaning should answer a question you will ask at review.
How many highlighter colours should I use?
Three. Enough to separate claims, evidence and terms; few enough to apply without stopping to classify.
Is highlighting in colour better than in one colour?
Only if the colours carry meaning you use later. A single colour applied selectively beats a rainbow applied everywhere.
Should I highlight before or after reading a section?
After. Read to the end of the section, then go back and mark. Marking on the first pass produces far too much yellow.
What colour is best for highlighting?
Yellow is the most readable over black text on white paper and on screens, which is why it is the default everywhere. Use it for the marks you will read most often.
Does colour coding work in digital highlighting apps?
Yes; most PDF viewers, e-book apps and web highlighters offer several colours. Map the same three meanings across every app so exports keep their sense.
Sources
- Dunlosky, J. et al. (2013). Improving Students' Learning With Effective Learning Techniques. Psychological Science in the Public Interest, 14(1).
- Colour Blind Awareness (UK): prevalence and design guidance
- Fowler, R. L. and Barker, A. S. (1974). Effectiveness of highlighting for retention of text material. Journal of Applied Psychology, 59(3).