Is methyl orange acidic or basic?
In an acidic solution, the protonation methyl orange takes place, as shown below



The deprotanted form depicted on the left is basic, and is yellow in colour. The red form is acidic and is depicted on the right. 

In general, a lot of acid base-indicators rely on the following mechanism of action

$$\ce{In^-}+  \ce{&lt;=&gt;[\ce{H^+}][\ce{OH^-}]} \ce{HIn}$$

At low pH, i.e high $\ce{H^+}$ concentration the $\ce{HIn}$ form is dominant since the equilibrium lies to the right, however, at higher pH $\ce{HIn}$ (which is weakly acidic) dissasociates to give $\ce{In^-}$ (its conjugate base).

Herein, $\ce{HIn}$ (the acid), and $\ce{In^-}$ (its corresponding conjugate base) have different colours. The colour changes are due to changes in absorption, due to change in the length of the $\pi$ conjugated system (as can be seen in the case of methyl orange pictured above).