Other studies suggest the colors are associated with the increasing content of lead, rubidium, and thallium ranging in amounts between 0.00X and 0.0X in the feldspars, with even extremely high contents of PbO, lead monoxide, (1% or more) known from the literature.
This passage suggests that the colors of feldspars are linked to varying levels of lead, rubidium, and thallium, with concentrations ranging from 0.00X to 0.0X.  The literature also reports extremely high PbO (lead monoxide) content in some samples.