Inspired by this thread on ture metallics, I decided to try a little experiment.
I took one of my painting holders (a wide platic bottle cap with a metal washer glued to it, and I use magnets on all my model bases as I transport them on metal sheets).
I cleaned the washer with steel wool.
I used masking tape (lightly pressed to one side) and primed one side with Tamiya white, then masked the other side and primed the other half with Citadel Skull white as I've found these to be the two best white primers.
For the metallic paints I brushed across the two primers. On the top half was GW Chainmail and the bottom half was VMC Gunmetal.
So three different painting applications as noted in this photo.
1. Dilluted the metalic paints with water only to a 50/50 ratio and did a couple of layers.
2. Straight out of the pot with a damp brush so the paint could spread evenly across both primers.
3. Using Vallejo Metal medium.
This photo is at an angle so you can better see the reflective nature of the paints. The GW paint is less sparkly than the Vallejo. For more realistic metals I would go with the GW paints for the base metal coloring. I don't know about highlights, I haven't studied that far.
The photos aren't as nice as I'd like them to be but this is my impression of the different paints on the different primers. GW is smoother, applies more uniformly and doesn't suffer from the metallic build up when dilluted by either water or the Metal medium.
Notes on the Primer.
Tamiya is more vibrant white, but also a little more glossy and water repellant. You'll fight with applying dilluted base coats a little more that with the GW primer. I've found from working with Tamyia that the primer has less chance to skunk out on you, but if you ensure low humidity, ~70degF temps and shake the holy bejesus out of any primer can, you'll reduce the chances of skunking.
Hope this helps. I don't have a tremendous amount of experience with true metallics (I've tended toward NMM) but I'm going to try True metal on my next project and this was a test application to see which one of these two I'd use. I'm going with GW dilluted with both water and a little Vallejo Metal medium.