Answer
Color is dependent on free electrons that are present for excitation.
Work Step by Step
The electron configuration of Sc is \[[Ar]4{{s}^{2}}3{{d}^{1}}.\] During the formation of \[S{{c}^{3+}}\] it attains noble gas configuration and no free electrons are present for excitation, so it is colorless. The electron configuration of Ti is \[[Ar]4{{s}^{2}}3{{d}^{2}}\]. During the formation of \[T{{i}^{3+}}\], its electron configuration becomes \[[Ar]2{{d}^{1}}.\] It possesses a single electron that can be excited by the absorption of light, so it is colored.
