All marks have a default color, even when there are no fields on Color on the Marks card. For most marks, blue is the default color; for text, black is the default color.
Dropping a field on Color on the Marks card, Tableau applies different colors to marks, based on the fields's values. The effect of color encoding on data view depends on whether the field dropped on color is discrete or continuous. Typically, dimensions are discrete and measures are continuous. For discrete fields, Tableau assigns a categorical palette, and for continuous fields, a quantitative palette.
Also use the Color drop-down control on the Marks card to specify other color properties such as opacity, borders, and halos.
Note: Color selections for a field are shared across multiple worksheets that use the same data source. To set the default color for a field, right-click the field in the Data pane, and then select Default Properties > Color.