The cheapest repaint for it would be to go for BR black, in which case you can repaint it from supplies at your local poundshop or B&M bargains (or whatever is similar and open), just grab a can of automotive primer (tends to be grey, but watch out for a rouge can of silver mixed in with the primers, I've made that mistake before) and some matte black. Depending on the stock some stores have other colours, my local poundland has had yellow, orange, green, blue, red, silver and white in the past so you may be able branch out into more colourful liveries.
Remove the body from the chassis. If you have the model I'm thinking of, it already has wasp stripes so using masking tape cover them up and any other areas you want to remain original. Then apply several thin coats of primer (give it some time to dry between coats), later on once the primer has dried apply the top coat in several thin coats again. Let the paint dry then remove the masking tape, add decals later if you want.
If the model doesn't already have wasp stripes apply primer to the entire body, then yellow paint to the areas you want to stripe, once that's dried use masking tape to cover up any areas you want to stay yellow and then apply the black paint.
Tamiya and some other companies make thin masking tape for modeling which is perfect for wasp stripes. I used 2mm masking tape and enitrely poundland spray paints on my 3D printed pannier tank body.
If you want BR blue then you're going to want to go down the pot of airfix/humbrol paint route for cheapness and use a brush as I assume you don't have an airbrush or want to buy railmatch spray paints, alternatively you can also buy spray paint for around £5 a can on amazon for more colour choice but your unlikely to get a perfect match to BR blue as you'd be guessing the shade from the image on a computer screen.