Yeah, I know what you're saying.
I tend to paint in small batches (2's or, if I'm feeling particularly dangerous, maybe 3's. Get this - I once did a batch of 4 in one sitting!).
To get that random look here's how I go.
Firstly, I add up all the areas of clothing that need to be done. So let's say they are all wearing leggings of some sort (I'll assume I'm painting 3 figures that day - so that will be 3 pairs of leggings). They all have an undershirt - so that makes 6 pieces of clothing) and an overshirt. This gives me a grand total of 9 areas of clothing.
I then think about my pallet. I decide I want an 'earth tone' look to them.
Now, I don't want them all to be the same, so I'm going to be using more than 3 different colours (i.e. I do NOT want them all in same colour leggings, same undershirt, same overshirt). However, using 9 different colours will seem too hotch potch. I want somewhere in between.
So I choose a couple of colours that will appear more frequently than others. This gives me a pallet of 5 or 6 colours - enough to avoid them all looking the same but enough to tie them all together.
Each model will have one or two of the key colours used on one (different) item of clothing. All other items of clothing will be painted from a random colour I take from my chosen selection of colours. As I paint I try to ensure that no two figures have all their clothing items in the same colours as another figure, or if that is going to occur that it occur on two figures who have completely different poses.
In this way you've planned ahead, so you know what colours are going to be used, but you've also avoided a uniform look.
That's how I do it anyway.