StartInstanceLocation, and SV_InstanceID

So, I'm trying to use SV_InstanceID as an extra input to a shader, to pick from a small set of vertex colors in code.

It seems to completely ignore the last argument of DrawIndexedInstanced(), and start at 0 per draw call. This seems less than useful, as it would make it impossible to transparently split up an instanced draw call, and defeat a lot of the purpose of having the system value at all.

How would one be expected to use SV_InstanceID properly in this case? The vertex shader looks about like so:

struct VertexInput
{
    float4 position : POSITION;
    uint instanceid : SV_InstanceID;
};

struct VertexOutput
{
    float4 projPos : SV_Position;
    float4 color : COLOR0;
};

VertexOutput vs_main( const VertexInput input )
{
    VertexOutput output = (VertexOutput)0;

    output.projPos = mul( float4( input.position.xyz, 1.0f ), g_ViewProjection );

    if ( input.instanceid == 0 )
    {
        output.color = float4(1,0,0,1);
    }
    else if ( input.instanceid == 1 )
    {
        output.color = float4(0,1,0,1);
    }
    else
    {
        output.color = float4(0.5,0.5,0.5,1);
    }
    return output;
}

This results in it always picking red. If I instead dig a color out of a separate vertex buffer, via D3D11_INPUT_PER_INSTANCE_DATA, it works as expected.

How do I make d3d useful?