this is why you should stretch watercolour paper!
If you use a lot of water in your watercolour painting, then it is important to stretch your paper before starting to avoid cockling/buckling - this is where the sheet wrinkles and forms ridges that are almost impossible to remove when substantial amounts of water is used.
However even more importantly, stretching your paper removes the size that is used on watercolour paper. Size is a light coating of a clear gelatine based glue, which makes the paper less porous and seals the surface fibres. This creates problems as the watercolour paint is repelled and will not give you a smooth finish.
Even if you are reluctant to stretch your paper than at least wipe it with a damp sponge it makes all the difference!
Below is a colour mixing exercise which highlights why you don't need loads of paint colours you just need red, yellow, blue and burnt sienna to start off with. Save your money and buy better quality paint but fewer colours.