That's really strange, did they give any reason for it? I've only ever used GW paints.

Vallejo paints are good, as are the Privateer Press P3 paints.  The latter is what I use personally (for the most part) and you can find some good equivalency lists online too, say at Bolter and Chainsword.  The Army Painter ones are okay too.  I've only just got some of them and so far have been pleased.

I dont see where Vallejo are complicated, apart from the fact that you can't open them like the GW, there just modelcolors like every other modelcolor too. I use them among GW and, said, P3 colors without noticing any differences.

You can also take a look @ the foundy color system: They have three color sets with a base color, a highlight and a darker tone alreday matched up.

I use Master Serie Paint from Reaper mini it's affordable, always fresh since you bought it directly from the manufacturer and you get free shipping for order over 25$us. I used it with GW citadel, army painter and Valledjo in the past and nerver have any problem. You can even go there and build your own set of colors. You should try them. They only thing it's there are in dropper bottle.

I'm not sure if this is not what you're looking for but they do sell a paint collection on their website.

https://www.games-workshop.com/gws/catalog/productDetail.jsp?prodId=prod1760002a

Here is what I use.  http://www.goldenpaints.com/  Golden Paints is a locally owned and operated company here in the U.S.A. that makes some of the highest (if not the highest) acrylic paints in the world. Their quality of ingredients and high pigment content of paint makes for a product which will last for a lifetime (end plug).

This page http://www.goldenpaints.com/products/color/index.php shows you the mediums you can get (air brush, fluid, heavy body) to meet your needs.  From the main page you can also explore the other products like textured pastes you can use on bases or other surfaces.

They do have 1 oz bottles of paint and larger.  At least this gives you another option.