In which case did the Supreme Court apply the doctrine of colourable legislation?

The Supreme Court has applied the doctrine of colourable legislation in the K.C. Gajapti vs state of Orissa case. The doctrine of colourable legislation implies that whatever is prohibited directly is prohibited indirectly also. This is intended to bar the legislature from doing something indirectly or covertly that has been prohibited from doing directly. 

Further Reading:

• Indian Judicial Doctrines Explained
• Polity Notes for UPSC