Brassy, multi-blonde w/ dark roots needs help (o:

the reason you are getting brassy is either because you are too dark or you arent leaving it on long enough. Blonde shades are only meant for levels 7 and lighter. Bleach is an option

You can leave that 40 volume on for 45 minutes, it should say that on the back of the dye bottle, but any longer than that won't help (it has done all it can do by 45 min.) but....

I'd suggest a test strand, because if your natural is dark brown, you probably aren't going to get enough lift with that dye and 40 volume.  (It might happen, you can try a test strand and see, I just don't think it is likely.  Sitting in a warm room instead of a cooler one will help a lot).  Most people who want to lift from dark brown to blonde use bleach from Sally.

Putting it on your roots first is a good idea, since they are so dark.  However, you are going to lift it to red with a dye that has a violet base, you need a blue or green base if you are going to try the 40 volume for lifting your natural color...if I were you, I'd just do bleach though, then toner, it will be easier.  The only hard part is you can't overlap or its too damaging, you might want to apply it with a brush. 

If you were going to go over your blonde hair though (like she said at Sally), you would want a violet base though, because violet counters yellow and it will make it less brassy.  (If you don't know why I say you need blue for one part and violet for another, read Claude's sticky at the top for a discussion of how the color wheel is important for hair dye).  You probably don't need to pull the dye through, unless you want to fix your brassiness.  With 40 volume, you will get a lot of lift on the already blonde hair though, it may go kind of whitish.  If you want to tone down the brassy, I'd do your roots, but with bleach, then think about maybe a toner or something for the blonde hair, as you'll want a violet toner for your new yellow roots anyway.