From Left To Right- (1-2 ) Grey (No dark factors) Normal (3) Grey (1 Dark Factor) Normal (4-5) Grey (2 Dark Factors) SF Yellowface I, Opaline

Effect Of This Color Adding Factor On Body: Grey budgies will be an even shade of light to dark grey body color depending on the dark factors the budgie has on their chest, breast and rump where it will be the darkest

Cheek Patches: Deep blue to silver grey

Main Tail Feathers: Black

Dark Factors: Grey budgies still have dark factors. For example a grey budgie with 1 or 2 dark factors can be noticeably darker than a grey budgie with no dark factors.

Single Factor And Double Factor Forms: There is no visual difference between a sf grey and a df grey. If you are unsure of their  back round the only way to know for sure is by breeding the bird in question to a normal (not grey) blue or green mate. If there are any normal blue or green chicks from this pairing the grey parent is a sf, as all the chicks would be grey with a df parent.