I must have mixed silicone at least 100 times and resin a 1000 times by now. Only a few months ago, I discovered the wonderful world of dye. Adding a dash of color has several advantages. First of all, it makes mixing easier. As long as you see streaks of dye or streaks of white in your mixture, you know that you have not yet stirred the mixture well enough. This is extremely important. If you do not mix both components of the silicone extremely well, you get pieces that do not cure properly. If you have a mold that has liquid pieces in it, you can throw it away and start all over. Furthermore, uncured silicone is next to impossible to remove from your original. It happened to me several times. There's a smiley for that :
A word of warning though : don't use too much dye. The chemical component of dye seems to influence the curing process if you use too much. I especially had problems each time I mixed blue and yellow dye together to make green molds or green resin. Reds and yellows work fine individually, and mixed into orange, too.
In the picture I used two colors, just for the picture. Of course one color is enough. A small tube of dye costs about a dollar (or euro), only a fraction of the cost of one ruined mold !!!
I use disposable equipment : plastic stirring-thingies and a cleaned yoghourt cup in this case.


It is stirred. Notice how interesting the use of dye can be. You'd NEVER see if it's properly mixed if you just stir the white silicone and its transparent hardener. BTW : you can actually SEE the hardener in the previous picture. It's a watery puddle on top of the white silicone.
As soon as you mix, you get air bubbles. You can avoid these if you have a vacuum machine. You can also avoid them by using a vibrator or (my method) gently tapping on the side of the mold container when the mold is poored. This makes all air bubbles come to the surface. Don't cheat when you make the mixture. Using more transparent component makes your mold cure faster, but it also gets harder and reduces the mold's lifetime. When you make it cure faster, you may not allow enough time for the air bubbles to float to the surface. This process can take over an hour. The mold needs four to 24 hours to harden before you can manipulate it.