It is very easy I decided to use 6 colors. So I divided the batter equally into 6 bowls, put in a squirt of color, mixed them and poured half of the first color into the pan, then half of the second color, etc. I reversed the color pour on the other layer. Pouring the batter in the center on top of the pervious color, forces the first batter to spred to the edge of the pan. If you wanted thinner bands of cake color, pour 1/4 a of each color at a time.( you would do the pour rotation twice for each layer.)
To answer two questions- my cake did not taste bitter or nasty from the colors used; nor did it leave colors stain your mouth. You really don't need that much color. A couple of drops was plenty.