What to make the paint on the walls lay down in an even layer

 The painting is uneven: stripes of various colors

Reason: if you added a kner and stirred yourself, then you did not make stirring very high quality.

 If the paint is tinted in a store in special tinting machines, maybe it also needs to be mixed well. There may be another reason if you had to tint the paint several times, may not fully observe the proportion and the color is slightly different.

Elimination: Mix the paint thoroughly using a drill. When tinting the paint, always accurately remember the proportions to make the same color if necessary. But it is better to do once with a margin.