it has to be the right thickness, but also when you're adding colours 2 and 3 to the first colour, try to pour it on so that there is very little height for the slime to fall.
eg if you have a tank with green gunge make sure the rim of the bucket of pink gunge is almost touching the surface of the green gunge before it comes out. This way it won't immediately soak in.
The easiest way is to split the tank into 2 or 4 sections with a separator. Our new tank has this so as soon as it warms up we can show you. We have done the layering before on sessions where we just made the Gunge thicker and poured it in gently. This does work as Mungo chutney can vouch for, not sure he'd want to put pics up though.
For those after Gunge , we have Natrosol 250 back in stock at £12.99 for 24 litres of thick, vividly coloured Gunge