I’m looking to buy food color powders and was wondering what brands give really nice vibrant colors. 

My favorites are Roxy & Rich and CK.

When you say "food color powders" what do you mean?  I ask this because there are many types of powdered colors, some of which are multi-use and others that are more single use.

There are very inexpensive powdered colors that are matte primary colors.  They are concentrated, edible lakes using FDA certified colors and you can use color theory charts to create custom colors: CK, Lorann, ChefMaster Powder colors                        Americolor Powder Food Colors

There are more expensive powdered colors that have already been custom mixed--some are water soluble and some are fat soluble, so you need to know what you want to use them for.  You can find those at http://www.roxyandrich.com    sugarpaste.com   &   thesugarart.com
to name just a few.