Acetic acid is an acidic medium. Recall that indicators are organic substances whose color changes in response to change in the pH of the solution. often times, the protonated and deprotonated forms of an indicator have different colors.
However, an equilibrium is set up when an indicator is in acid/ basic medium. Methyl orange is red in acid medium and yellow in basic medium.
Hence while in acetic acid, the equilibrium lies towards the protonated form of acetic acid, hence the solution appears red.
