We check each statement one by one.

Option A: $RuO_4$ is an oxidizing agent.

• Ruthenium tetroxide, $RuO_4$, is analogous to $OsO_4$ and contains Ru in a very high oxidation state, $+8$.
• Species in very high oxidation states generally act as strong oxidizing agents.
• Therefore, this statement is correct.

Option B: $Cr_2O_3$ is an amphoteric oxide.

• $Cr_2O_3$ reacts with both acids and bases.
• With acids, it behaves as a basic oxide: $Cr_2O_3 + 6H^+ \rightarrow 2Cr^{3+} + 3H_2O$
• With strong bases, it shows acidic character and forms chromite-type species.
• Hence, $Cr_2O_3$ is amphoteric.
• So this statement is correct.

Option C: $VOSO_4$ is a reducing agent.

• In $VOSO_4$, vanadium is present as vanadyl ion, $VO^{2+}$.
• Oxidation state of V: $x + (-2) = +2 \Rightarrow x = +4$
• Thus vanadium is in the $+4$ state.
• Since vanadium can be oxidized from $+4$ to $+5$, $VOSO_4$ tends to act as a reducing agent.
• Therefore, this statement is correct.

Option D: Red colour of ruby is due to the presence of $Co^{3+}$.

• Ruby is essentially $Al_2O_3$ containing traces of $Cr^{3+}$ impurity.
• The red colour of ruby is due to $Cr^{3+}$, not $Co^{3+}$.
• Therefore, this statement is incorrect.

Since the question asks for the incorrect statement, the answer is: $\boxed{D}$

Comparison with stored correct answer:

• Stored correct answer: $D$
• Derived answer: $D$
• They match.