[1] Open [Options] in the [Menu] of 3D Scene.
[2] In the Options dialog, select Colors from the list on the left.
[3] [Color] → [Set Color for], select [3D Curve].
In the Color field on the right, choose the desired color.
To specify a color other than those displayed, click [More Colors] and define a custom color.
[4] After setting the color, click [Apply] and then [OK] in the lower-right corner of the Options dialog to finish.
Setting the Color for an Individual 3D Curve
[1] Select the 3D curve.
[2] Switch to the [Properties] browser.
[3] Clear the check box for [Default Color].
[4] From the Color drop-down menu, select the desired color.
To specify a color other than those displayed, click [Other] and define a custom color.
[5] After setting the color, click in the scene to apply and confirm the change.

When multiple 3D curves are selected, color cannot be assigned.
You must set the color for each 3D curve individually.