The two theories (propounded by Harlow & Rawlings, Law and Administration, 1984) represent opposite philosophies about the proper role of administrative law:

• Red Light Theory views administrative power as a potential danger to individual liberty. It emphasises control through strict judicial review by the ordinary courts — law as a brake on administration. It is associated with Dicey and the common-law tradition.
• Green Light Theory views administrative power as a positive instrument of social welfare. It emphasises facilitation through internal mechanisms, political accountability and good administration — law as a catalyst. Judicial review is seen as a last resort.

Thus the key difference is: Red Light = control-oriented, external, judicial; Green Light = welfare-oriented, internal, political.