Actually I like Satin Ice gumpaste the best and it takes colors very well. And yes it does harden, unless it is in extreme humidity and heat the shape should dry and retain itself. Then have fun and approach it like you were a kid playing with Play Doh. 
But, if you're short on time, you could make your own gumpast and save money. Gumpaste is easy to make and hardens just like the commercial brands. Here is a recipe found here on CC:
Ingredients
1 Lb Powdered Sugar Fine -10X plastic bag
1 Packet Knox Gelatin
2 TBS of Lemon juice
3 TBS of Water
1 Tsp of Glucose
Instructions
Add gelatin, water & juice over double boiler; dissolve about 5 to 7 minutes. Add glucose stir. Use a mixer with dough hook. Place sugar in mixer then add liquid mix until slightly together remove and knead. If it feels too sticky add a little more sugar. Wrap with plastic wrap and place in plastic bag, seal. Let rest over night.
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