Universal indicator solution and paper. This indicator, available in both liquid and paper-impregnated form, is used to measure the strength of an acid or an alkali. When the paper is dipped into a solution, it changes colour depending on the solution's pH. Colours that correspond with different pHs, from low (acidic) to high (alkaline), are shown on the wheel at upper right. Yellow is neutral (pH 7), whereas orange and red are acidic, and green and dark blue are alkaline. Similar colour changes occur when universal solution is used. Acid solutions have a high concentration of hydrogen ions, while alkali solutions have a low concentration of hydrogen ions.