An electric field can be visualized by drawing field lines, which indicate both the magnitude and direction of the field. Field lines start on positive charges and end on negative charges. The direction of the field line at a point is the direction of the field at that point. The relative magnitude of the electric field is proportional to the density of the field lines.
Where the field lines are close together the field is strongest; where the field lines are far apart the field is weakest. If the lines are uniformly-spaced and parallel, the field is uniform.
Another way to visualize field is to use field vectors, which are uniformly spaced. This conveys the message that every point in space has a field associated with it. The field vector at any point gives the direction of the field at the point, and the color of the vector shows the strength of the field.
In the simulations we use the colors go from black (strongest field) to red, green, blue (weaker field) and then gradually fade away as the field decreases in magnitude.