Cobalt Blue Deep, which uses a distinct pigment from Cobalt Blue (PB28), is a highly granulating, purple-toned deep blue similar to French Ultramarine (PB29).
Gradient: Highly granulating with some visible strokes. Deep purple-blue to pale periwinkle with visible large granules. I really can’t tell this color from French Ultramarine.
Transparency: WN calls this semi-transparent, but I can see quite a bit of blue on the line so I would call it semi-opaque. This potentially distinguishes it from Ultramarine, which is transparent.
Glazing: Nice glaze to a darker and purpler blue.
Color Mixes: WOW, look at this granulation! It’s absolutely fabulous in every mix! As with the other properties, very similar to French Ultramarine, but if anything even less mixable with even more outstanding floating granulation.
Comparison to Other Colors
Cobalt (PB28) and Ultramarine (PB29)
Cobalt Blue Deep is much more purple-toned and granulating than a typical Cobalt Blue (PB28). In my estimation, CBD is a bit greener than French Ultramarine but purpler than non-French Ultramarine, and is as extremely granulating as the most granulating Ultramarine.
Compared to Ultramarines, CBD is notably more expensive. In Winsor & Newton’s line, Cobalt Blue Deep is a series 4 color, while either Ultramarine Blue is series 1. For me, that means that currently CBD is over $20 for a 14ml, while Ultramarine Blue is under $15. That can add up, especially if it’s a color you’re buying pretty frequently. I would need a more compelling marginal improvement to justify favoring it.
Cobalt colors are also more toxic than Ultramarines, which generally isn’t a big factor in my decision-making but it might be something to note.
My Overall Review
While I don’t see a ton of reason to favor this over Ultramarine, it is certainly a good option for an alternative. I like it as well as my favorite French Ultramarines. That said, the practical concerns (particularly price) have me leaning more toward Ultramarine for my palette.