I don't see you're issuing a clear (which is a huge red flag unless you're doing it on purpose and know what you're doing).
Perhaps you need to enable RenderDoc's save initials setting.
RenderDoc also allows you to check the entire pipeline, see the outputs of the VS, and even debug the VS and PS shaders. Have you tried that?
There's also a pixel history log that will tell you why a pixel is of that colour (e.g. it was cleared, then set to red by pixel shader, then rejected a pixel shader due to depth buffer, etc)