Litmus is a dye made from lichens. It is red in acidic solutions, blue in basic ones and purple in between, changing over a range from about pH 4.5 to 8.3.
Red litmus paper turns blue only in a basic solution, and blue litmus paper turns red only in an acidic one. In a neutral solution neither paper changes.
Litmus shows whether a solution is acidic or basic, but because it changes over such a wide range, it cannot show how acidic or basic the solution is.
Each indicator changes colour over its own range, about 1 to 2 pH units wide. Methyl orange changes from red to yellow at about pH 3 to 4.5, and methyl red from red to yellow at about pH 4 to 6.
Bromothymol blue, often called BTB, is yellow below about pH 6, green around 7 and blue above about 7.6. Phenolphthalein is colourless below about pH 8 and pink above about 10.
Outside its range, an indicator shows the same colour whatever the pH, so the right indicator must be chosen for each purpose.
An indicator is itself a weak acid, written \mathrm{HIn}, whose molecule and ion have different colours. In water it ionizes partly, giving \mathrm{H^+} and \mathrm{In^-}.
In an acidic solution there are so many hydrogen ions that most of the indicator stays as \mathrm{HIn}, and its colour shows. In a basic solution, hydroxide ions remove hydrogen ions, so most of the indicator becomes \mathrm{In^-}.
For BTB, \mathrm{HIn} is yellow and \mathrm{In^-} is blue. At about pH 7 the two forms are present in similar amounts, and the mixture looks green.
BTB solution adjusted to green, about neutral, turns yellow when breath is blown into it through a tube.
Exhaled breath contains much more carbon dioxide than air. The carbon dioxide dissolves and reacts with water to form carbonic acid, which gives hydrogen ions and makes the solution weakly acidic.
The same indicator shows that water plants take in carbon dioxide in light: yellow BTB solution around a plant turns back to green and then blue.
Universal indicator is a mixture of indicators that changes through many colours across the whole pH range. Paper soaked in it, pH test paper, gives the pH when its colour is compared with a chart.
A pH meter measures the voltage of an electrode dipped in the solution and shows the pH as a number, usually to 0.01 or 0.1 of a unit.
For 0.1 mol/L ethanoic acid, test paper gives about 3, while a meter reads 2.9. Test paper is quick and cheap, but a meter gives more exact values.