Re: Demachy and red chalk
For what it's worth I mention that with mined colors like Venetian Red and Red Chalk, there really are no absolutes. For instance, I used a particular "Venetian Red" in acrylic for years, and when it ran out bought a new jar same brand, same name, but noticeably different. The "explanation" (given but not verified personally) was that that particular mine (in Italy if memory serves) had run out & the pigment was now coming from a different mine.
Manufactured colors (ultramarine blue, for instance) are, given the same company, more likely to be constant... but even so, no guarantee.