Here is the approximate effect I'm describing.
It's a linear gradient that has been "circlized". I have Photoshop, Inkscape, or can just notepad some raw svg.
Even just knowing what term to Google for would be useful.
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This effect can be achieved by many different ways like masking and etc but I'll use photoshop's Angle Gradient effect to achieve this one.
Watch below video tutorial if you feels stuck!
This is for Inkscape
Begin with a circle
Select the gradient mesh tool, in the tool controls bar hit the Conical gradient button, set 8 rows and 8 columns, and click and drag on the circle to apply a mesh.
Click and drag to select all stops on the gradient, and change the colour to green
Select other stops, and change to white
Click and drag the arc controls on the circle so you have just one small segment visible.
Note that gradient meshes aren't supported in SVGs at present. A workaround for this would be to rasterize the mesh object, and place it inside a clipping mask.