It's worth keeping in mind that the MTG color wheel doesn't directly relate to morality the way D&D's alignment chart does. White is the color of order, society and valuing the collective over the individual, which I think makes perfect sense for Lae'zel and the Githyanki. Sure they're cuthroat, viscious and murderous, but it's all in the interest of creating a strong society, killing Mindflayers and furthering the goals of their queen. In a way, the Githyanki are the worst example of white philosophy, short of being a forced hivemind; the individual holds no value beyond what they can offer to society, and so if they're weak enough to be killed, then their killer is doing right by society, removing weakness from it.
Similarly, Black isn't just the color of evil, though it certainly is the most associated with evil things. It's the color of ambition, self-interest and power whatever the cost. Which I think makes sense for a cleric of Shar.
