For myself, I like to work off of a mood board or style sheet, made specifically for VFX. I usually make one myself, show the team, get alignment on the goals, and then reference that throughout.
As far as what I put into a mood board, it’s usually divided by shape language and color. For the shapes It’s often images of other game FX, or any super rough sketches that I make. The other part is color, and I usually find a handful of images (of anything really) that represent the spectrum that I’m shooting for. A quick ramp in Photoshop can go a long way
If there is a concept artist on the team it might be worth talking to them about the style of the effects, they might be able to help you out, possibly even sketch out some ideas for you if they have time. It’s definitely a luxury to have that help, but not always necessary if you have good reference!