it's actually really easy, just spray half of it as normal (i did grey first) then hold the spray can of the other colour quite far away and start to blend between, if it's coming off as not a very smooth transition then hold the can further away, don't try to build it up all in one, just build up the effect over many coats... I even ended up spraying grey over the blue side to make the blue not as bright. you could always practice on a piece of cardboard first

 thanks bud, great advice i'll try that asap
happy riding man