Painting - Quest for the perfect highlights

Finding the perfect colours for the Sentinels proved rather difficult. Since there is an army to be painted I had a few principles in mind before I started to select:

1. A shade a base colour, a highlight and an extreme colour must do, there will be no more levels
2. No drybrush on the armour - on smooth surfaces I never seem to get it right...
3. Layering is preferable - Because that's the way I like it
4. No full-body washes

I had dark blue in mind, and since I'm a great fan of Regal Blue, originally chose the following colours on my trial marines:

0th variant - Original
Shade - VMC Dark Prussian Blue
Base - Regal Blue
Highlight - Enchanted Blue
Extreme highlight - 50% Regal Blue- 50% Ice Blue
I thought that it was going to be fine, but it turned out rather lame. The highlights were too crude, and I didn't like the result at all. So I went to the bits box for my leftover metal torsoes and I knew I have some serious testing to do.
Please note that I did not aim even at TTQ on the samples below, only wanted to find the most suitable colours using minimum time and effort. Here are the results

1st variant
Shade - VMC Dark Prussian Blue
Base - 50% VMC Dark Prussian Blue - 50% Regal Blue
Highlight - Regal Blue
Extreme highlight - Enchanted Blue

Actually I liked this variant, it had contrast without too extreme highlights

2nd variant
Shade - VMC Dark Prussian Blue
Base - Regal Blue
Highlight - 50% Regal Blue - 50% Enchanted Blue
Extreme highlight - Enchanted Blue

A little disappointment. While I like regal blue, this combination was a bit too low contrast.

3rd variant
Shade - VMC Dark Prussian Blue
Base - Regal Blue
Highlight - Enchanted Blue
Extreme highlight - VMC Pastel Blue

.... as this one is too much. Pastel Blue is a tad darker that the Regal Blue -Ice Blue combo I used earlier, but still too light.. At this point my wife being the one person jury chose the first variant for further improvement.

4th variant
Shade - none
Base - VMC Dark Prussian Blue
Highlight - 66% Regal Blue 33% Enchanted Blue
Extreme highlight - Enchanted Blue

5th variant
Shade - none
Base - VMC Dark Prussian Blue
Highlight - 50% Regal Blue 50% Enchanted Blue
Extreme highlight - Enchanted Blue

Since the base colour is very dark as it is, I dropped shades, and used only the highlights, which is good since it will speed up painting a bit. Perhaps I will use black liner or Badab Black ink, we shall see.
To cut a long story short Patricia and I agreed that the 4th variant looked best. Not too harsh highlights but exciting. a bit crimson fist-like look. Or Dark Angels in blue.

Do you have any tips on how could I further improve this scheme before I start painting my first tactical squad?