OK, so I am making icing for a practice cake I am going to start making tomorrow. I wanted to try a different icing and a lot of people on this site say they use 1/2 butter and 1/2 crisco to frost the cake. Now if I want the icing on the cake to be white I can not do this correct. Silly me as I started making it did not even think, dah the butter is going to make it yellow. It is not stiff so I am not sure I will be able to use it for decorating. What do I do with it?
As it is, you can probably use it to ice the cake, just not for decorations. The reason it's too thin is probably too much liquid. The amount of water/butterfat varies with the butter you're using, and so does the color. Generally speaking, the cheaper the butter, the higher the water content, and they usually have more artificial colors in them too. Even Borden & Land O'Lakes are not that great. In the future, you may want to user a higher quality butter to get a whiter icing. European butters are generally whiter, have a higher butterfat content, and will overall give you a much better end result. Unfortunately that comes with a higher price tag.
Also, when you make any icing using butter, especially if you're using a new brand for the first time, add your liquid a little at a time so you don't end up with soup. I made that mistake the first time using store brand butter. When I made it with a higher quality butter, I wound up using all the liquid.
Good luck.
Ali 
You can also just add more icing sugar until the icing is the consistency you require
I use the 1/2 butter 1/2 crisco recipe with Land O'Lakes butter. I find that once it is all mixed, it still turns out white enough for me. Once you're finished making it, you may find you can use it to ice the cake after all.
here is a recipe for you and is great with viva towel. It is easy to smooth
3 cups shortening (crisco or hi ratio)
2 lbs poweder sugar
1/4 water
2 Tsp. flavoring (you can 1 tsp each of vanilla and almond, or 2 tsp of butter
or lorana oils are good. your choice)
cream shortening and sugar together. then add water and flavoring.
this frosting can stor for months because you have no dairy. It is very white.
good luck
Wilton has a white you can add to whiten your icing. Since you've already made it and it is a practice cake, I say give it a try. You could probably do as MissBaritone suggested and stiffen it up.
Note: Using butter will also alter your colors. Not much, but because your not starting with white, they will be alittle different.
Rose
I prefer the taste to wondering how white it looks. My customers love my BC & I use half and half
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