From all the rules in the COLREGS, rule 1 and 2 are most difficult to interpret. Let us see what rule 1b and 1c says.

Rule 1b

Nothing in these Rules shall interfere with the operation of special rules made by an appropriate authority for roadsteads, harbours, rivers, lakes or inland waterways connected with the high seas and navigable by seagoing vessels. Such special rules shall conform as closely as possible to these Rules. 

This rule is allowing the local authorities to have any special local rules for the navigation but these rules shall not interfere with the rules in the COLREGS.

For example, in Singapore Strait, the MPA Singapore has made a rule which requires all the ships crossing the TSS in the night to display three all round green lights in a straight line.

Rule 1a allows the authorities like MPA Singapore to have these rules. Three all around green lights is not there in any of the other rules in the COLREGS. So this rule does not interfere with any of the COLREGS.

Rule 1c

Nothing in these Rules shall interfere with the operation of any special rules made by the Government of any State with respect to additional station or signal lights, shapes or whistle signals for ships of war and vessels proceeding under convoy, or with respect to additional station or signal lights or shapes for fishing vessels engaged in fishing as a fleet. These additional stations or signal lights, shapes or whistle signals shall, so far as possible, be such that they cannot be mistaken for any light, shape or signal authorized elsewhere under these Rules. 

This rule is with respect to the war ships or ships proceeding under the convoy.

For example,  the ships proceeding in a convoy for transiting the Gulf of Aden may assign certain sound signal in case of a pirate attack. These sound signals, however, shall not be different from any of the sound signal required under the COLREGS.

Same is the case with war ships. The government of a country may have sound and light signals for instructions to the navy ships. Rule 1c requires that these sound or light signals should not be similar to the ones in COLREGS.

So while the content of the rules 1b and 1c is similar in nature, 

• Rule 1b is for any special rules made by the authorities for anchorages, harbours, rivers, lakes, and inland waterways
• Rule 1c is for any special rules made by the government for warships, ships in convoy or fishing vessels in convoy.