Hi Rohit Thank you for your help.
Hi Mariano,
You need to use a ColorFunction to do that. e.g.
ColorFunction
ParametricPlot3D[{t, t, t}, {t, 0, 3}, ColorFunction -> Function[{x, y, z, u}, If[Between[x, {1, 2}], Red, Green]], ColorFunctionScaling -> False]