Matt is the opposite of glossy or shiny. You can make acrylic paint look matt by using a medium specially designed for that purpose. Acrylic matt medium is a standard paint additive that you can mix directly with your acrylic colours. In addition to rendering a matt finish to your paint, it also extends the flow of the paint, making it easier to apply. The matt medium won't affect the colour. It looks white when it comes out of the container, but mixes colourlessly with your paint, and dries to a translucent finish that leaves your original colour unchanged.
Choose the colour of paint you want to look matt, or mix the colour on the palette using the palette knife. If you are mixing the colour, mix it well. The colour should be uniform.
Add an equal amount of matt medium as you have paint. Use a cup for mixing a large amount. This quantity of matt medium makes the colour incredibly matt. It is better not to exceed 50 per cent matt medium at first, as you may make your colour translucent. Some colours and paint lines mix differently, based on the type and quantity of pigment. Experiment a bit before exceeding the 50 per cent matt medium quantity.
Mix up the medium well. Use the palette knife or the brush if you are mixing in a cup. The matt medium stirs or mixes in easily, but make sure the mixture is all one colour.
Apply your mixture to the canvas or painting. It dries in about an hour, depending on conditions. High temperatures and low humidity mean quicker drying times.