This is an example code to reproduce a browser bug that happens 100% of time in Firefox and a bit less often in Chrome.

1. Draw a shape with the red color.
2. Click on the fill button, then change the color to green or blue. Click on the drawn shape to fill it.
3. Repeat for the rest of the colors.

All colors are drawn solid and it's easy to fill them with another color.

Some colors (like the red color here) are drawn with a border or shadow. Drawn shapes, are not of solid color and can't be filled with another color.