You can put your clay pieces into either a cold or a hot oven, although putting them into a preheated oven may make it easier to gauge the firing time. This gives the underlying material more time to expand and contract with the changes in temperature, making cracking of the clay less likely. It's a good idea to use an oven thermometer to make sure you have the correct temperature, since toaster ovens often have a lot of variation in temperature. In general, the color of polymer clay stays the same after being fired. There are a few exceptions to this.
Some reds turn darker after firing.
Fimo art translucent may also darken if fired at the higher temperatures. If you're in doubt, you can fire small samples of the colors you're using to see what the end result will look like. The new clay will stick to the pre-fired clay, making a strong bond.