You can colour it with normal gel colours. Some suggest candy colours, but my experience is that gel colours work well.

The cupcakes in my photos with the yellow buttercream is actually Passionfruit flavoured Italian meringue buttercream. I didn't use any food colour, the colour was purely from the passionfruit. Have you considered something like that?

Ohh! I just reread your post and you don't want it yellow lol...

A couple of options..

1) use lighter butter, in the UK we have a brand called Lurpak which is practically White
2) add a tiny bit of violet food colour, less is more approach! The violet neutralises the yellow.
3) get a White food colouring, we have a colouring called superwhite, not sure if you have it in the US

Well after reading all of your tips and recommendations I am going to have to try SMBC.