Are mixtures always coloured substances?

No, LSD is a clear liquid, which is why people are caught walking around with it in their water bottles. But maybe you mean when two substances are added they change colour. To understand this we have to know what gives colour, which is electrons. They absorb light and release it again in different wavelengths. read more

No, a mixture is not always a coloured substance. The colour that we see due to mixing is generally due to the reaction that takes place between the solvent and the solution but there a some exceptions as well which don’t change colour on reaction. read more