I'd say - turning turquoise silk into teal is easy. If it"s a light enough turquoise, just overdye quickly in a teal green. I wouldn't even bother with yellow. If it's a darker turquoise, I'd probably try with a pure green - what is that called, emerald? I would start with a diluted dye bath for overdyeing (i.e. at most half of the dye called for in the recipe), rather than using a full strength dye. You can always do it again and add color, removing color is possible but more difficult.
And, acid dyes -
www.dharmatrading.com is the way to go, they have instructions and dyes.
I have overdyed silk before - and it worked quite well.
You can also undye dyed silk - there are special non-bleach chemicals for that, available from Dharma Trading.