Anyone know of a totally 'white' white chocolate- all I can find is 'yellow'

The candy coatings made with all vegetable shortening and artificial vanillin will be "whiter" than real white chocolate (ivory tint due to real vanilla).  Look for Wilton brand if possible (Merckens white wafers are not quite dead white).

As you have surmised, the titanium dioxide will also help with the "whiteness".

But there is a limit.  You could try formulating something with the hardened coconut fat from canned coconut milk (chill in fridge to separate) and pure icing sugar and white vanilla.  That would be the "whitest" food.

Over here in the USA we use Merkins Wedding White chocolate.

To whiten "white " chocolate you need to add titanium oxide to it. 

And while titanium dioxide isn't exactly "Good Eats," it is, so far as anybody's been able to determine, biologically inert, and therefore so long as it's sanitary, and not contaminated with anything toxic, it's perfectly food-safe.

I use merkens white white chocolate and it is white, not yellowish. hth

For richer colours in modelling chocolate, get food colour in powder form, or oil based colours that are specifically formulated for chocolate.  Look at www.Lorannoils.com for information on how the formulation is different. These powders are NOT the dusting or pearl powders sold for gumpaste work--thgey are the concentrates that gel and paste and other food colour liquids are mixed from.

My local bulk foods has Merckens candy wafers in several colours which blend very well with Merckens white. MUCH nicer than other brands.

You can premix the powder colour with the coconut oil to make it easier to blend.