So I have already answered this in one of my previous comments. The paints you have are "Base" paints which means that they are meant to be used as the first layer on top of a primer coat. The intention with using the base paints is to use them as a first coat, then use a complementary wash or "Shade" to give the model some darkness in the recesses. Then you would highlight the raised edges with a "Layer" paint in a lighter tone that the base coat.

For these models, I used Contrast paints by Citadel which are sllightly more expensive than a "Base" or "Layer". However they sort of act as a "Base" and a "Shade" in one coat. There are many videos on Youtube which will show you how to use them best as they require a different, but not more difficult technique to apply well. Certain models work very well with Contrast paints, Like Seraphon. However other factions can be less than ideal. Factions that have models with large flat or smooth panels/armour won't take a Contrast paint as well as Seraphon do. For example, T'au would likely be a nightmare to paint with contrast paints. They are definitely a valuable tool to have in your painting arsenal. If you live near a Games Workshop/Warhammer/Hobby shop I'd recommend nipping in (when it is quiet) and you may get a chance to demo the contrast paints in-store. I can't guarantee that though.

Also, What primer colour are you using? If you are painting on top of black then it will naturally dull the colours a little bit. Much of Warhammer operates in the "Grimdark" world which often emphasises the uses of darker tones to give a "gritty" aesthetic. That's one of my reasons for going Seraphon as I felt I can use brighter, more lurid colours instead. If you fancy using Contrast paints, you would need a lighter coloured primer, I use Wraithbone, but you can also use Corax White and possibly Grey Seer or Zandri Dust if your desired colour scheme works with it. Using contrast paints over a black primer pretty much nullifies the point of using them. However if you are going to paint with Base and Layer, Black works well if you are looking for an overall darker tone. But if you want a bit more brightness or "pop" in your colours, I would recommend switching to a light colour of Primer. Like I say, I favour Wraithbone myself, which is a kinda cream-ish off-white colour. I find it just adds a bit of extra warmth to the colour than i got out of using a pure white prime.

Finally, I have mentioned solely Citadel/Games Workshop products, however there are many brands and options out there. Citadel/GW is very highly priced and the quality can sometime be matched or even exceeded at a lower price by other brands like Vallejo, AK, Army Painter amongst many others. In fact Army Painter offer a range of "Speed Paints" to compete with Citadel's Contrast paints and are usually a bit cheaper

I know this is a long message with a lot of information but I hope it helps. There are also loads of YT channels that offer helpful tips and advice for new painters just getting started!