A coloured liquid, such as felt-tip pen ink, may look like a single colour, but is really a mixture of solids dissolved in water.
It is very hard to separate out the various substances because they are dissolved. However, you can see what they are made of if you get the coloured ink to seep up a porous substance, like a filter paper (the kind used to filter coffee), because some colours will seep up the paper faster than others.
