So number #16 is has too much warmth (orange).

I was going to counteract with blue (ash). 10 vol.

And #613 is pale yellow, I want to make it significatly darker.

Do I need to fill? With what?

I was going to use a violet to cancel out the yellow.  

If you are just toning down the #16, I think using a demi ash color with a blue base is a good idea.

As for the #613, how much darker do you want it?  If you want to keep it still blond, but just darker blond, then you need to fill it with a gold filler, process that, and then use your target color.

If you want to color it to a medium brown shade, you would want to fill it with orange, and then use your target color.

For fillers, if you have access to Schwartzkopf products, they have filler sprays in gold, copper, and red where each color corresponds with the underlying pigment  of your target color.  For example, if you are wanting a target color in the range of 7-9 use the gold filler, for 4-6 use the copper, and 1-3 use the red spray.

Other than those sprays, I know most manufacturer's have 'recipes' on how to mix the appropriate filler for the your target level.

What color line will you be using?

be careful and try with one strand only- the pigment might just deposit and not the color u are wanting. you might end up with mud colored hair.

i once and ONLY once colored some 613 with redken o9nb, i wanted to tone out the yellow and make it more beige~ oh yeah- i ended up with GREY hair, had to do a quick bleach wash and then it came out, but man what a mess! oh and no i didnt do just ONE strand, i did all of them applied in clients head, NEVER again would i color extension hair and this was really really good hair too.