Primers basically have four functions: 1. seal very dry,porous walls 2. act as a stain killer 3. provide adhesion to a slick surface, and 4. facilitate color change. You are concerned with the last two.

In the case of really sllck surfaces, many would prefer an oil primer, followed by either a latex or oil paint. However, the newer water and acrylic based primers are capable of sticking to slick surfaces. Zinssr 123, Behr No.75, and Glidden Gripper are examples of these.

If a significant color is to be applied to the wall, it is common practice to tint the white primer to almost the color of the finished color. This additional tint will increase the covering ability of the white primer, plus, any light bouncing back outwards will be the appropriate color.

Gray primers are usually used under either very dark colors, or under very bright colors ( red, yellow, green, blue) that have very poor covering ability. By bright colors, gray is beneficial because gray does not bounce light outward, as does a white primer.

Thanks for your replies, her landlord brought today a 5-gallon pail of white Dulux Gripper primer. Her bedroom will be a dark teal/aqua, so we will get a gallon of tinted primer for this room, the rest will be rather neutral colours.

Should we use white Gripper then a tinted regular primer on top, or get a gallon of Gripper tinted aqua?

One coat of primer, whether tinted or white, is sufficient. After priming, go with your paint itself. If it does not cover, go around again with the paint. The idea of tinting the primer is to possibly get by with only one finish coat of paint.

As long as the primer is "high-hide" (extra blocking pigments) you will be fine. I did a "Dr Seuss" house - everything was in bright primary colors - with Glidden's high-hide white primer as the first coat and it was fine. Just make sure you are rolling a nice even coat and not skimping.

The use of GREY primer is when you are covering a light with a deep tone - it minimizes the white show-through.

If your paint color is mixed from a "white base", you are just as well off using two coats of the paint itself, unless there are other problems to deal with, such as high gloss paint on the wall. When doing this, your "primer" is the same color as the finish coat. A full bodied white paint will have just as much white pigment as a white primer. It is also a time saver as you don't have to deal with two different materials and additional clean up. In certain scenarios, it is no more, or even less expensive than dealing with two different materials, especially from a pro painter's perspective where time is more expensive than materials.