All cellulose fibres such as cotton need to be treated with a substance that is high in tannins, before the mordant step.  And unfortunately almost all tannins will leave at least a small amount of colour on your fibres, usually a brown/beige/pale red shade.
We use gallnut in our kits, because we find that it is the tannin that leaves the least amount of colour.  But it will still leave a little.  With gallnut, it tends to be beige, and it is much more likely to happen if you leave it overnight.  This is not a problem, as the dye colours that you add to the fibre later are much stronger and will completely cover it.  But if you are doing tie dye and want to leave a bit of white showing, you are better off soaking the item for only 1 hour, rather than overnight.  It will still shift colour slightly, but much less noticeably, and once the colour is on, you won't notice it at all You can still leave it to soak in the alum overnight - that step will not change the colour.
But rest assured, you did not do anything wrong!   The beige colour will be easily covered up by the dye colours, and usually serves to enhance and deepen the shades in a beautiful way.