How do I do the different blue shades like this on a BC cake?

I would mix up a batch and add in the colouring, taking a little out of the bowl every so often so I have separate portions of different shades of the same colour ( or in reverse, make up a part of a batch as dark as I want the darkest and then add in increasing quantities of un-coloured buttercream in stages to get the different shades).
 

To mix them together for the uneven look of this cake, I'd just take my spatula and put a daub of each colour here and there and then smooth them together a bit with the spatula to get a more even spread and gradation.

Start with the lighter color as the base coat then add on a darker blue, then a darker until you have the graduated colors you want. Good luck!

Thanks guys. I figured I would do the base coat in the light blue, then just add darker colors around and smooth together. I am just worried it will look messy.