gray should work fine.if you plan to blocksand several times,talk to your paint jobber about adding a slight amount of color (3-5%) to your primer.blocksand it till you start to burn through.prime it a second time with a slightly different 'tinted' primer.block sand again.this will show you 'highes/lows'.repeat until car is straight.IMPORTANT...prime a solid color and prep without burn-throughs before painting basecoat.
ps.you can also apply a trace coat with cheap aerosol paint prior to blocking if primer isn't tintable.good luck,thx,cj.