I've been working on this came for past 3 days. My customer will be coming to pick it up at 11am in the morning. I just need to put a layer of ivory white buttercream on one of the tiers and unfortunately I think I added a little too much of yellow (I searched and found that mixing yellow and brown in white gives you white). I don't want to add too much brown either. That will give me more of a beige hue. 

Anyways right now I stuck everything in the fridge as I'm too tired. Any advices what do I do to fix the color? I tried adding about a cup of confectioner sugar but it hasn't made much of a difference. Please Help!!! 

Yellow and brown do not make white! You'll just make a lighter brown which wouldn't be bad in a very small amount to make ivory. Violet  neutralizes yellow, but mixed with the brown I don't know what it would do. Try a small amount of buttercream and add a tiny amount of violet and see what happens.

Have you got any white food colouring?  If not, I'd probably be inclined to just to make a new batch of buttercream and save the yellowy one for something else. SMBC comes out pretty much ivory without any colouring added...  what sort of buttercream are you doing?

Hi guys, 

thank you everyone for the amazing advices!!! So I made a new batch and just added a wee bit of buttercream from my yellow batch and it made the perfect color the bride wanted. 

I just love this website and forum. I made sooooo many mistakes and had so many questions and concerns but did not know who to ask. I really am thankful to you guys and this forum! ❤️

Looking forward to seeing a pic of your finished cake!   :)