Thank you Jacob, I'll try these suggestions, I think it will be a redraw tbh..
Katie Lois, I believe it is worth considering this way (thin stroke no fill, Smart Guides on if needed):
1) Create a rectangle the size of the object and align it so it is placed right on top of it;
2) Create a circle with the same height, then left align it with the rectangle so its leftmost Anchor Point is on tip of the curve;
3) Switch to the Scale Tool, set the leftmost Anchor Point as the centre, then ShiftClickDrag by the rightmost Anchor Point for best fit to the oval shape of the curve; adjust further if needed (there may be different ways), and you can lock the rectangle while you are at it;
4) Select both rectangle and oval and use Pathfinder Divide + delete leftmost bits + Unite, or use Shpae Builder/Live Paint, so you have one path to recolour.
Edit: this came after the post by Monika below, impossible to see tomorrow in Threaded View.
