These two are closer than most brand face-offs, so let me cut to it. Mylands is the older, denser, more pigment-heavy of the pair. It's the paint that supplies a lot of the West End theatre and film world, and you feel it on the wall — colours have a real depth and a slight sheen even in matt. Their range runs 211 colours with proper strength in neutrals (40), greens (32) and greys (25), and an LRV range of 4 to 91, so they cover deep and pale equally well. A green like Acanthus Leaf No.12 has that smoky, lived-in quality that's hard to fake, and Alderman No.60 is a gorgeous moody blue that earns its keep in a study or dining room.
Paint & Paper Library is the more conceptual brand. Its signature is the Architectural Colours system — five tonal strengths of the same hue, designed so you can paint ceiling, walls, woodwork and detail in graduating shades and have it all hang together. That's why their names carry Roman numerals: Slate IV and Lead II are points on a deliberate scale, not random shades. The range is 186 colours, strongest in greens (34) and neutrals (33), with whites that go right up to a near-pure LRV of 99.2.
So here's the honest split. If you're doing one room and want maximum richness and a true period look, Mylands wins — the pigment density just reads as more expensive. If you're doing a whole house or a layered scheme and want the woodwork, walls and ceiling to relate properly, P&PL's tonal system does the thinking for you.
Both are premium-priced and both behave well under the brush. My practical tip: order A4 samples or large peel-and-stick swatches from each, tape them up on the actual wall, and judge in morning and evening light before you commit. The brand matters less than the colour looking right at 6pm in your room.