When you begin to create an idea for a mosaic in your head, it's very hard to conceptualize what color glass and grout best fits for your final product.
Even if your composition has definite colors. For example Sunflowers obviously only come in yellow, but what type of yellow is right? That's up to you as an artist. I personally am heavily influenced by Van Gogh, so my palette would consist of earth tones. For example, I wouldn't use a bold cadmium yellow, I'd use a golden rod with possibly some accents of sienna or brown.
The best way to begin is to come up with an idea,and make some rough sketches on paper while doing so. This is a very important step in the design process, so that you don't leave out any brilliant thought that can make your art one-of-a-kind.
I as an artist, like to waste a lot of time trying to perfect the process of my work by bringing the concept of my idea to a sketch and then to a "final drawing". Because even though I have a sketch, the "final drawing", allows me to make any last minute changes so that I have a great sense of direction.
The "final drawing" is essentially a well drawn template or illustration in ink. I save them for the future, so that I can revive the piece and/or approach it with different colors and technique. -joshy b