Follow these steps:
- Once your shape is created on your first keyframe, copy it over to another frame at the end of the duration of your color fade effect. Remember: do not make this into a symbol. Your tween will not work if you do.
- On your second keyframe, select the line and fill, and then move the Alpha slider in the Color Mixer panel to the right all the way down to 0%. (If you only want to make it semi-transparent, set it to a higher number.)
- Select the range of frames from your first to your last keyframes.
- On the Tween dropdown menu in the Properties panel at the bottom of the screen, select "Shape".