Hi Gauri.
It is tricky to get rid of the yellow tone in highlights. Yellow highlights often blend nicely with lighter brown shades, but they don't look good on level 2 or 3. The highlights will need to be toned. The easiest way to do this is to tone while the highlighting cap is still on the head (so the toner doesn't affect the base colour). Or, if you have applied the bleach with foils, rinse off the bleach and then apply the toner to the highlights only, using foils to protect the rest of the hair.
Do you have access to Clairol Born Blonde Toners? They have blonde shades designed to be used on bleached hair. A shade like Born Blonde Winsome Wheat will impart a warm beige tone to yellow highlights. Precious Platinum will impart a silvery tone. Your clients can then maintain their tone by washing their hair with a purple shampoo like Silver Brights, Clairol Shimmer Lights or Artec White Violet. They may have to come back to you occasionally for you to refresh their tone. This is done by using the same toner, but omitting the colour balance crystals that are used with the first application. Here is a link to information about Born Blonde Semi-Permanent Toners -- http://www.beansbeauty.com/search/index.php?control=simple&q=born+blonde+semi-permanent+toner&x=14&y=11 - http://www.beansbeauty.com/search/index.php?control=simple&q=born+blonde+semi-permanent+toner&x=14&y=11
If you don't have access to any true toners, you can use a violet-based semi-permanent blonde colour to get rid of the bright yellow colour in highlights. Try to apply the colour only to the highlights, though, because most semi-permanent dyes contain peroxide which will lighten the base colour. Some semi-permanent options are Wella Color Charm, Igora Color Toner, Clairol Natural Instincts and L'Oreal Castings ColorSpa. These are only a few of your options. You may have different brands available to you where you live.
I didn't understand your first question. Did you mean that you WANT to give someone GREEN highlights? I don't know what to tell you; that is something I try to avoid. 
I hope some of this is helpful to you. Best of luck. 
------------- My user name is WAY too long. Just call me Juliana. :-)
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