the white is stronger than the Grey. Why?

[HerenIstarion]

Haunting Spirit
Posts: 96


They (all of the istari) were restrained indeed. they were forbidden to use their power openly, but their quest was to gain trust of elves and men, treading on equality, not by the splendour of their might, and re-kindle the courage and will to resist the Evil
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they...were messangers sent to contest the power of Sauron, and to unite all those who had the will to resist him, but they were forbidden to match his power with power, or to seek to dominate Elves or Men by force or fear

UT, Istari, p 409

...being embodied the Istari had need to learn much anew by slow experience, and though they knew whence they came, the memory of the blessed realm was to them a vision from far off, for wich (so long as they remained true to their mission) they yearned exceedingly. Thus by enduring by free will the pangs of exile and the deceits of Sauron they might redress the evils of that time

UT, Istari, p407