What a rubra form is?

Dyckia delicata rubra form - above
Dyckia delicata - below

Rubra form is the same as dark or reddish form.
It is stable, durable, everlasting reddish form.
Many Dyckia species has a rubra form.
This is not a red phase, a red phase is a temporary , instable and induced by dryness, heat, light or fertilizers. Removed the cause the plant gets bach to its original phase.
The rubra form is perpetual, it is forever and never induced by any means.
Some Dyckias exist only in the rubra form and some just the green one and some exists in both form: green and rubra.
The rubra form tends to be much more beautiful and so most sought after.
Some rubra form is just olive or a bit dark not even getting red our reddish.
In a heavily scaled plant as this Dyckia delicata the red or rubra form is almost not visible but look again. You will see.