I have a big problem differentiating one unit from the next when they are exactly the same between units. If you notice in the picture above, it's rather easy to tell each unit apart since you have the green ones, the grey ones and the purple ones. It will be extremely confusing once they are all in the same colour scheme.
I am really open to suggestions to identify each unit from each other.
Here are a few things on top of my mind.
I do not want to use a strong accent colour which will distract from the original colour scheme (purple - Hive Fleet Leviathan). A complementary colour is probably the best. I am thinking of putting a dot or two along the creatures' tails. One dot for one unit, two dots for another unit. The potential problem arises when I need more models for unit one whereby I will have to paint more Gaunts!
Any other suggestions?