Many acrylic colors will need to have some gall added to them to make them spread well. Marbling gall is a surfactant, or wetting agent (actually a type of detergent). It makes the colors float and spread out on the surface of the size. After you start marbling, if you find that some colors are not spreading well, a drop or two of gall should be added to them. The gall should be added into your paint, never directly into the marbling tray. If gall is not used, a lot of the color may sink to the bottom of the tray. Some colors require more gall than others; some don't need any at all. This is figured out mainly by trial and error: if a color does not spread well or sinks, add one drop to the color and test it; then another if necessary, etc. The gall is pretty powerful; use only one or two drops at a time.
The weather has a lot to do with whether a color needs gall or not. A change in temperature, humidity, or air pressure may cause some colors to start sinking or spreading too much, even if they were working fine before. So before each marbling session, you should test each color by sprinkling a few drops of paint on the size to see how it is responding. If the color sinks or only spreads out a little, add a drop or two of gall into the paint. If it spreads too much because of too much gall, add more paint to the jar. Do not put gall into a full, new jar of color — it is easy to get too much. Always work out of a separate jar with only a small amount (a half inch or so) of color in it, and add your gall to that.
This may all sound a bit complicated, but it isn't -- the whole concept of using gall is simply this: if a color isn't spreading enough, add a drop of gall. If it still doesn't, add another drop, etc.
Marbling gall gets its name because marblers used to use actual gall from the gall bladder of a cow. When working with watercolors or gouaches, many marblers still use real oxgall. But real oxgall does not work with acrylics. The marbling gall we sell is not an animal product.
Gall is also necessary in making the Italian Vein and Overmarbling.
Any time you want one color to dominate over all the other colors, that color should have a small amount of extra gall added to it to make it spread more than the others, such as the dark red on the top example here, or the blue in the examples here.
And if you want several different shades of the same color, such as on these Stone Marbles, that is achieved by applying a color to the size, adding a drop of gall into your jar of color and applying more (which will make it a bit paler), then adding more, etc.