I don't think there is a standard key, but you can assign one via Tools->Samsung text message for computer mac. Customize keyboard., select Format in the left column, locate and select FontColor in the right column, then assign a key (make sure you actually click the Assign button. Then, if you also want a 'set to Auto color' keystroke, select the Color: command in the Format list.

A color dropdown will appear at the top right of the dialog box. Ensure Auto is selected (or select the color you want) and assign a keystroke to that. – user181946 Oct 3 '13 at 19:31. Although it's not exactly what you want it can set the text colour to the automatic colour of microsoft word.