Acidic solutions consists of two water and acid, the water dissolving the acid, which comes in solid form, such as citric acid, or gases.
The measurements of pH of an acidic solution is done through the use of an indicator, which is a substance consisting of one compound or a mixture of compounds, that modifies it's color when they are added to acidic or alkaline solutions.
Methyl orange is used as an indicator in titration of weak bases with strong acids. The color of methyl orange is red at pH 3.1 and it turns to orange-yellow at pH 4.4.
Methyl orange color changes are the result of the way electrons are enclosed in a molecule when hydrogen ions are disconnected or connected. Methyl orange molecules absorb green light, making the solution to appear as having red color.