Pre-Heresy Emperor's Children

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That purple looks great man, you did a really good job. I love the gold too, overall I think you did a great job - it definitely needs "something" to make it pop, you're right. I think finishing the base, and doing the eyes, will help tremendously. It also wouldn't hurt to do one more highlight of straight genestealer purple, just on the sharpest corners/edges at the top, to break up the model a bit more and give it more definition. Just like a dot on the corner of the crest of the helmet, the corners of the armor on the shins/arms, and along the hard edges of the top part of the backpack, would do wonders I think.

But also adding the transfers will make him pop too, so don't forget about that!

I don't know if you're trying to stick to a very "by-the-book" or very specific spot color scheme that only allows the gold on the trim of the shoulder pads, but if not, I would do more gold accents. It's not really an accent yet as is. 

When I stick to a minimum 1 or 2 colors in art, plus neutrals like black and white, you have your primary show up the most (purple) and your secondary (gold) should show up in at least a few other smaller places - at least 3 different parts, even if they're small, to show that it's purposeful and coherent.

If you look at the stormcast that are posted here, they are Silver primary but, the Crimson secondary shows up in multiple spots followed by bronze. The really good necron schemes posted here are a good example of using a neutral (metal/gray) as your primary and then having two secondary to play with (teal, gold). You'll see on some models the teal is the secondary and the gold are the accents, some models it's the other way around. But all of them have both of those accent colors repeated in a couple places to pull it together. Same goes for the scheme that's metal and then Red and Green. These are posts in the Painting Logs section of this site

This is just one theory/idea, not a right or wrong by any means. I would personally love to see more gold and would also be very pleased to see really bold whites worked in to some models like you said - whether it's their shoulder pads or their heads. 

Also, as always, basing goes an incredibly long way in changing how a model looks because you can introduce a brand new, very bold color to the entire piece.

I've seen some AWESOME emperor's children tacticals that strike that balance between "standing out as a legionaire of the EC" and "but also this is a mass produced armor set and so should be a little more plain" very well - and I think you're extremely close to that mark.

I would say that there are 2-3 things that really help - painting 1/both knee pads gold as well, painting the top plate of the backpack gold, and if you feel like it (and don't feel like its too much) painting the face gold (like just the part with the slits in it below the eyes).

That, and the big white III for the 3rd legion and a golden EC logo on the shoulder pads, and you'll have an amazing tactical on your hands in my opinion.

Another theme I saw was painting the studs gold rather than the kneepad. What would look better for some regular tacticals? Trying to be extravagant while leaving room to add more gold with sargeants/vets.

I like this a lot better man, this is exactly what I envisioned when I described it. I think it's the right amount of accent color in several different places. 

If I had to choose, I'd do the knee(s) gold instead of the studs based on aesthetics alone but I don't really know if those things signify anything for marines.

Have you considered using green to paint the eye lenses?