Dyes are organic compounds with absorbances in the visible range. Some of these, such as methyl orange or phenolphthalein, will shift their conformation slightly in the presence or absence of hydrogen ions. This shift causes a change in the absorbance maximum of the compound and hence a change in color. As a solution drops below about pH 4, methyl orange turns from yellow to orange. As a solution goes above about pH 9, phenolphthalein goes from clear to pink.
Mixed indicators show a range of colors in response to pH changes.
You can get pH indicator strips. These are strips of paper or other material on which dyes have been fixed. When wetted these will show a particular color corresponding to the pH of the solution. A color chart is used to read the strip. A chemical cell consisting of an acid-permeable glass membrane separating two solutions will develop a voltage related to the difference between the hydrogen ion activities in the two solutions.