Changing settings for date symbols 
Use
You can change the color, size and position of the date symbols in the graphic area of the Gantt chart using the planning board assistant.

You can change the symbols themselves in Customizing for Basis.
Features
Dates are displayed in the Gantt chart as bars or the start and finish dates are represented by individual symbols. The system gives dates that belong together the same color. For instance,
For example, if you change the color of the forecast start dates the system also changes the color of the finish date and the date bar.

The color, pattern, and values such as Edge type and Edge width, are used for the dates that belong together. Vertical offset and Element height are an exception to this. Here you can change the values for each date.
You can use a pattern and consequently a background color for most date symbols.
The date symbols for the start, finish, and actual dates are simple circles, whose line color (edge color) you can change. This means that the background and foreground color as well as the pattern have no effect.
Activities
Define the colors for the dates in planning board assistant.