I turned on the Lock Fill option of the Paint Bucket tool, but when I fill a shape with a gradient, I just see a solid color. What's going on?
If you think you're looking at a solid, it's possible that you're just seeing one end of the gradation. The best way to use the Lock Fill feature is to first fill a shape with Lock Fill turned off and then turn it on and continue to fill other shapes. Otherwise, what often happens is that the gradient's Fill Transform handles are set so that the falloff is extremely large. If this happens, you can try setting the Stage zoom level to 25% and selecting the Fill Transform tool. You might then see the handles for the gradation way off the Stage.
What is the best color specification system to use: RGB, HSB, or Hex?
Whichever one you prefer. No one system is better than the other. For every color onscreen, there's a corresponding color value in any of the systems. If your client provides its trademark colors in RGB, you should use RGB. If someone provides you with the hexadecimal values for color, you should use that system. The three systems are practically the same.