The lyrics are often taken as speaking for themselves but as so often the locations carry a lot of meaning, as comments here point out:
"As to the bells themselves the bells of Brecon and Wye are off the s Wales coalfield, which meant that they could be happy without the mass unemployment endured by the coalfield to the south. Neath is in the anthracite coalfield of s Wales, which didn't experience the depression of the 20s and 30s so severely. Caerphilly is just off the coalfield. Many miners from Merthyr and Rhymney traveled to pits in the Neath valley at that time. At this time the British government was talking of closing down Merthyr and transporting the people to England- hence the brown bells."
"As for the phrase "Throw the vandals in court" this isn't a reference to the coal owners. Because this cry came from Newport an area which suffered less unemployment in the 1920s than the south Wales coalfield, the cry to put the vandals in court refers to the militant miners of 1926, as they were seen as vandals who refused work and created social disturbance. From Swansea onwards Idris Davies is portraying the social unease felt by the more prosperous areas towards the south Wales miners."
The bells aren't agreeing with each other, they are voices in conflict (though Davies might also have been remembering the uprising of the Newport Chartists a century before). Here's the original poem (the song cuts the Brecon line in order to repeat the Rhymney line as chorus):
Gwalia Deserta XVO what can you give me?
Say the sad bells of Rhymney.Is there hope for the future?
Cry the brown bells of Merthyr.Who made the mineowner?
Say the black bells of Rhondda.And who robbed the miner?
Cry the grim bells of Blaina.They will plunder willy-nilly,
Say the bells of Caerphilly.They have fangs, they have teeth
Shout the loud bells of Neath.To the south, things are sullen,
Say the pink bells of Brecon.Even God is uneasy,
Say the moist bells of Swansea.Put the vandals in court
Cry the bells of Newport.All would be well if — if — if —
Say the green bells of Cardiff.Why so worried, sisters, why
Sing the silver bells of Wye.