I think its more along the lines of the Void creatures are rapidly evolving/adapting. The void from which they came either has a different form of electromagnetic radiation that allows its inhabitants to see causing Voidborn who come to Runeterra to have issues with the UV of Runeterra burning them since they're not used to Runeterra's light spectrum causing them pain and discomfort (Vel'Koz says I miss the darkness of the void, Cho'Gath complains about the Light Burning, could be literal pain or could just be the same experience one has when turning on the bathroom light after leaving their dark bedroom, blinding almost painful brightness since their world has no light the sudden introduction of light would likely be painful to their visual organs). This sudden introduction of light could also explain why most Void creatures are purple/black but the ones Diana fought were white, lets look at moles and other subterranean organisms that live in little to no light conditions, what happens to them? Their eyes become small and none functional or they get evolved right off the organism entirely since light is not needed in their environment to function, they also become pale since they don't need a defense against solar radiation that pigmentation serves.
We even see this pattern in humans, humans in darker, more overcast areas that require wearing lots of clothing (England, Iceland, Ireland, etc...) all have pale skin (even the Irish/Scottish Highlanders, some of which wore only kilts (until the weather forced them to dress warmer) and fought naked/barely clothed in combat. This pale skin is because they didn't need as much protection from the sun and needed to be able to absorb enough sun to make vitamin D. Compare this to Africans who have darker skin and live in much drier, arid climates with long sunny days that don't require inhabitants to cover up to keep warm (in fact it can be downright uncomfortable to be totally covered in hot climates). Their darker skin protects them from the sun's harmful rays (there's a reason those of African descent don't burn easy) but also is less efficient at making Vitamin D since they got more consistent sun and didn't need to have the lighter skin with maximum Vitamin D production capabilities. Void monsters could be the same way, in their Dark dimension they don't need protection from the light's affects because there is none and since there's no light being pale isn't an advantage or disadvantage so they stay pale so they don't waste energy producing pigment. In Runeterra there is light so they need to make pigment to protect themselves and provide camouflage against the background. We already know Void creatures evolve very fast I mean look at Kha'Zix his main goal is evolution and Cho'Gath he is able to grow very quickly, so we already have established that Void creatures evolve fast and are able to reorder their physiology to adapt to varying circumstances.
The other thing I can think of is that the Void creatures react differently based off of perceived threat, like when the void monsters first appeared before Diana, (it was never said they came from some form of portal, just that their energy was polluting the area) they were white because they didn't see her as anything more than a snack so they weren't in combat mode, after she cut two down as if they were butter sculptures the third turned purple and black and had hooks/barbs/claws/spikes erupting from its flesh, making its self better suited for combat in the low light conditions (darker colours to better blend in and disguise movements against the dark background, while additional natural weaponry in the forms of more spikes/claws to better combat its clearly formidable foe)