On GameCube, the analog AV out port has separate pins for composite, S-Video, and RGB. This is a holdover from SNES. The main issue is NTSC titles disable RGB and enable S-Video. (Swiss intercepts this and keeps RGB enabled.)
On Wii, S-Video and RGB share the same pins. Specifically, S-Video's Y pin (luminance) is the same pin as RGB's R pin (red), which is why you get a red image if you use an RGB SCART cable with an NTSC title.
It *should* be possible to force a PAL60 video mode to allow use of a Wii RGB SCART cable with NTSC games. Some USB loaders might have an option for this. On the other hand, YPbPr (component) is a better choice, mostly because it supports 480p.