How can you partially recolor a Stroke/Outline?

i want change color of stroked shape partially (see attached image):

 

i tried using circle as  clipping mask red rectangle. when using clipping mask, red rectangle clipped fill of object, not stroke in step 3. there way keep stroke , not expand stroke own shape can later change stroke 10 points 20 points, while retaining partial red coloring?

 

i tried making 2 stroked circles on separate layers , coloring/clipping top 1 having 2 strokes stacked creates jagged edges instead of smooth vector look. top stroke never covers strokes below, regardless of color mode. example dublicating green stroked circle , changing color red should make disappear on red background, parts of green circle still show through:

 

thank you!

loriot,

 

one way to:

 

1) copy circle in front , and give desired colour,

2) create rectangle on top,

3) select 1) , 2) , object>clipping mask>make.

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