I don’t have PY150, but I do have PY128 (Rembrandt rather than WN). PY128 has high chroma, is a very yellow-yellow, and is very transparent. Rather low tinting strength.

I looked up PY150 in a database, and it seems to be less transparent, slightly leaning orange, and of less chroma.

In short, they are two different things. I use PY128 when I wish to make a slight hue adjustment in that hue range. It is very good for that purpose. It might be OK for those who do CMYK painting with transparents, but it does not have enough horsepower to modify colors such as the cadmiums, and is easily overwhelmed by the phthalos.

PY128 and PY150 (Nickel Azo Yellow) are very different. PY128 is a high chroma, transparent bright yellow – a good choice as a primary mixing yellow. You can add Titanium White if you need it to be more opaque.

Nickel Azo is even more transparent which is why you it looks so deep and brownish in masstone, but is a moderately-high chroma middle yellow in undertone and tints. It’s like two colors in one (very duo-toned). It might be considered the darkest yellow. Great for dark & deep mixed greens and other mixes.

Neither of them are particularly strong in the oil medium. Nickel Azo Yellow is stronger than Azo Condensation, I think, and it’s cheaper. From experience, I’d say they’re more useful as glazing colors. Everything overpowers them, it seems like. That’d be why nearly everyone’s oil palette contains cadmium yellow, yellow ochre, and maybe some form of naples yellow.

PY128 isn’t PY150 by any means. PY128 is far brighter and greener. It’ll mix some outrageous greens. PY150 brightens to a handsome yellow, but nothing high chroma. It’s cheaper in the long run if you buy Winsor and Newton’s.