Fimo Classic is known for it's ability to firm up very quickly after it has been conditioned. While some clays are O.K. to condition one day and use the next, Fimo Classic is best used right when it's been conditioned. MixQuick will help - you can use about 1 part MixQuick to 3 parts Fimo Classic. It will not change the color of your clay.
Sealing in anything won't make a difference because it's not drying out. Polymer clay does not dry out. When it gets softer or harder it's a function of heat, friction, and distribution of the materials that are used to make the clay. While using just heat (pocket, heating pad) will help, it's the friction of beating or rolling that really gets it soft enough throughout to work with.