"At an art show I found some vibrant paintings using a thick medium, lovely and bright and glossy, applied with a knife, most artistically. I haven't a clue what medium it is, only that it stands out from the canvas, almost a plasticky look. Could it be a colored Artex? Please could you help me in the correct direction to try to emulate it?" -- Jill
It sounds like it was acrylic texture paste or molding paste, which is a type of acrylic medium. This is formulated to mix with acrylic paint without changing the paint's color, and being much stiffer than paint you can really sculpt into it with a painting knife. It's a bit like seriously stiff peanut butter. You can also paint on top of the paste, as with any other acrylic medium.
Although texture pastes are white, they don't change a color like white paint does (they don't have white pigment in them). Some pastes dry clear and some dry white. Both will reduce the intensity of a color a bit, depending on how much paint you mix into the medium, and may make transparent colors opaque. Do a test before you start to see what the results are, before you start on a 'real' painting.
All the major art brands produce acrylic texture medium. Check the description to see how thick it is, and whether it can be carved into or sanded when dried. This is useful if you find you want to change something when it's dried.
If the final painting isn't glossy enough for you, a layer or two of gloss varnish will help. Just be careful not to pool the varnish around ridges in the paint as you apply it.
Buy Direct: Acrylic Texture Paste