Normally I'd say Silver, since my EchoHatch right now is Silver (actually, more like Grey); even though my favorite exterior colour is Black, I'd say Silver would be the safe bet because of what ECHOKnight said that Silver hides dirt pretty well. So unless you're gonna religiously clean and wax your car, Silver saves you the hassle.
Having said that, with the amount of mods you're gonna put it to make it look more like a racing car, I voted for White since that colour associates more with track racing cars (even though the Yaris sedan is far from being that). And the contrast given by the White and your carbon-black mods will really stand out. I'm not a big fan of White on cars (except for Porsches

) but I agree with hasher22 that the Toyota White looks really good right now.
There, I made your choice for ya TOYOTaejien

Good luck with the purchase and take some pics as well (for us that is

)
EchoHoLiK
PS: You got some mad Photoshop skills

Can you also do one in Black, in Red and in Metallic Blue? Just curious as to how they'd look as well. Thanks.