Double clicking on the line icon will bring up this panel.
Linestyles, thickness, symbols and symbol sizes are controlled with this panel.
The current color is shown in the color well. Clicking in the well will bring up the color selection patch.
To change the color of the coasts lines just choose the desired color in the patch and drag it into the well. Please note that no color patch will follow the cursor, only when you release the mouse in the well will the color appear.
As an alternative to removing the coasts is to select the hide radio button. If a layer is hidden it will remain in the layer manager but will not be plotted.