Creating the quilt top and piecing things together.
This is the foundation and primary focus of your quilt project. When you join seams you potential create ridges from your seaming and the thread that are holding the pieces together. For this reason a fine thread is appropriate. A good start would be a 50 weight thread that will blend into the seams when pressed flat.
In a quilt that is archival quality it is important to use 100% cotton thread as this will out last your polyester threads if you are using cotton for your quilt top. The polyester can potential wear down the cotton fabric and tear through it.
Quilting the pieces together.
When selecting a thread for the quilting (bringing your sandwich of top, batting and backing together) use a heavier weight thread, say a 50 weight, to create strength and durability in your finished piece. When doing hand quilting the threads made for this are generally coated....in old days women often ran their thread through a cake of beeswax to give the thread strength. Do not use thread made for hand quilting in your sewing machine. The coating can potentially gum up your machine, which is not a good thing.
Whether you are machine or hand quilting the selection of color can make a huge difference in the final quilt. If you want your quilting to blend, use a neutral color like a light grey or something that is a predominate color in the quilt top and back. If you want high contrast use a complimenting color thread that will stand out and be noticed for the color it adds to the entire quilt.
Regardless of the type of quilt you are creating, take as much time and effort in selecting the right thread type, weight and color to ensure the best results possible.