Something like this ?
The path is made from the desatureted picture : Colours / Desaturate / Mono mixer. Then use the 'fuzzy select tool' to make the slection and convert the selction to a path : Select / To Path. Then make a new transparent layer for stroking the path and blur the stroked path : Filters / Blur : Gaussian Blur. For changing the colours, from the gimp menu : Colours / Hue-Saturation use the 'Hue' slider to change the color on the color change layer.
spiderman.xcf (Size: 895.16 KB / Downloads: 91)
The path is made from the desatureted picture : Colours / Desaturate / Mono mixer. Then use the 'fuzzy select tool' to make the slection and convert the selction to a path : Select / To Path. Then make a new transparent layer for stroking the path and blur the stroked path : Filters / Blur : Gaussian Blur. For changing the colours, from the gimp menu : Colours / Hue-Saturation use the 'Hue' slider to change the color on the color change layer.
spiderman.xcf (Size: 895.16 KB / Downloads: 91)