Unlike dyes, which produce the same color every time the same amount of dye from the same lot is added to a batch, iron oxides are pigments, which are not soluble, so the color comes from suspended oxide particles, rather than from solutions of individual dye molecules.
The intensity of the color of an iron oxide suspension depends largely on the number of iron oxide particles suspended in the product. For any given weight of iron oxide pigment, the color will intensify as the iron oxide particles are ground finer and made more numerous.
The issue is complicated by the fact that iron oxide particles agglomerate (stick to each other) readily, so not only do you have to grind/de-agglomerate the particles, you also have to make sure that they do not re-agglomerate in your product.
Pigment wetting can also be an issue - a pigment particle that is completely wetted will appear darker/more intense than one that still has air bubbles trapped on its surface.
In order to get consistent batch-to-batch color using iron oxide pigments, you have to do four things:
1) weigh your pigments very precisely
2) grind/mill/homogenize them into your batch exactly the same way every time
3) Use the right amount of wetting agents, suspending agents and anti-agglomeration ingredients
4) adjust your pigment amounts every time you get a new lot of pigments