Odd Atari 2600 power supply issue?

Had a very odd issue recently.

I make up power supplies for retro consoles (SMS/MD, NES/SNES, Atari etc) and the people I get the power supplies have changed either the design or where they get them from, which means I had to do a re-test on all the consoles I do them for.

Testing the new power supply (switch-mode 9Vdc, 1.3Amp) on MD, SMS, NES and SNES all proved fine. When testing on my AV modded (simple transistor version) 2600 jr I had a black and white picture. I use the older power supply for my modded 2600 with no issues.

What I tried though was I trimmed the colour trim pot in the 2600 jr and I got colour again! Whoa weird! I did some simple measuring.

On the old power supply no load (not that it matters since they are regulated) I read 8.8Vdc, the new power supply read 9.2Vdc. I also did a rough (very) AC ripple test and got 180mVac on both.

So

1/Old style switch mode power supply Atari 2600 colour is fine.
2/New style switch mode power supply = b/w picture
3/adjust colour trim pot on Atari 2600 with new power supply = colour is back.
4/colour is now fine on both power supplies.

Thoughts?