make a multicolored line entity in AutoCAD

MLINE functionality can be used for this. Use the MLSTYLE command to define a new multiline style and define (element properties) two line elements, both with the offset equal to 0. Set different colors to them (e.g. green and red). Use different linetypes, e.g. DASHED and DASHED2 (you may need to Load them first) - for real-life applications you will probably need to define your own linetypes. Save the new Mline style. Now use the MLINE command to draw a polyline using this new mlstyle - it has alternating green/red colors!