What you demand unfortunately is not available in Mathcads plot. There are two workarounds which come to mind, none of hem being perfect.
1) In the plot format menu you can chose tab Appearence and change "Color Options" to colormap. In tab Advanced you can chose a colormap and also what the map should be related to. This is usually the height (z) but there is no option to use the order of the points for that purpose.,
2) You plot ever point separetly and format its color (and size) individually.