Matrix SoColor 507N/508N: Addressing Brassiness & Grey Coverage

Question
I'm using 507N from Matrix So Color and my grey is coming out brassy without good coverage. I'm thinking the level 8 would be too light for me but is there a reason why 508N wouldn't turn out as brassy on my roots as the 507N? Also, why is it not covering my grey? I'm using 20V developer from Matrix and letting it process for 45 minutes. For some reason the front of my hair is also pulling darker. My hair seems to be the same texture all over so I'm puzzled why the front pulled a little darker. Any suggestions? Thank you in advance...

Answer
Sorry for the delay.
Matrix So Color has a tendency to not cover greys that well. And the reason why different parts of your hair are turning out different colors and tones could be because your hair has different types. Um i cant really decipher why the front seemed to be a darker color then the rest. The developer i recommend using is probably a 30 vol. from Oreor. Probably process it for about 55 min., try to apply the dye to the roots first and add a tad bit more to the roots, letting the roots process more effectively.
Have any more questions? Something i didnt cover? Ask/tell me again.