There are several ways to color and prepare your icing. This is just the best way I have found that works for me.
I begin with a stiff icing. I like to mix my icing in these Ziploc plastic round containers. I like that they are round so they are easier to mix with a spatula and they come with twist on lids so easy for storage. Because I use these containers all the time I know that a full container of icing will flood approx. a full tray of cookies.
I like to start with a small amount of color on my spatula. I can always add more if needed. Tip: If you need a light or pastel color put icing on your spatula and put just a tiny dot on your spatula then start mixing.
I always color my icing with outline/piping consistency. If I am making a large batch of one color ie orange at Halloween or red at Christmas I will mix my color in the mixer. I let the paddle attachment do all the hard work for me. I will add the gel color directly to the icing when it gets to an outline consistency.
I love to use tipless bags for all of my icing, both outline and flood icing. I like that they are clear so I can easily see the color and they are disposable, so I just throw away after I use them.
I also like to use a tip with my tipless bag for all my outlining. Here is a post about why I do this. If I’m not going to be using my icing with a tip right away, I will put a damp paper towel over the tip. This keeps your icing from crusting over and clogging your tip later.
I prefer Americolor Food Gels and have recently started using The Cookie Countess gel colors and I really like them. Gel colors store very well and it’s easy to add just a drop to you icing.