Recently we spotted a subtle problem with the Animations Editor, if you click on any of the color swatches with their default value of transparent black, that color will be set to be used to draw.
If you try to pick a different color from the main color picker, C3 preserves the alpha value, but because it is now 0, the new color remains fully transparent. Further more, C3 also preserves the luminance of the old color when you pick a new one, in this case the luminance is 0% because the old color is black. This all means that after the color is set to transparent black it makes it seem as if you can't change colors!
This has been fixed for the next beta release, and shouldn't be too long before it lands in the next stable release.
In the mean time, make sure the alpha value is different to 0. You can set it to 255 for a fully opaque color.
Also make sure to check the position of the luminance slider (right below the main color picker), if it is all the way to the left, the color will always be black, if it is all the way to the right the color will always be white. The middle will give you the same color as the one picked in the color picker.
You can also change the luminance of a color by changing to HSL color space (the very bottom of the color palette). The you can change the luminance value of the color manually, it ranges from 0% to 100%.
Hope that helps.