Works for me. MtoA 4.2.1.1
<?xml version="1.0"?>
<materialx version="1.37">
<look name="ramp">
<materialassign name="materialassign1" material="ramp_aiStandardSurface1_material_shader" geom="/pSphere1/pSphereShape1" />
</look>
<material name="ramp_aiStandardSurface1_material_shader">
<shaderref name="aiStandardSurface1" node="standard_surface">
<bindinput name="base_color" type="color3" nodegraph="NG_ramp_aiRampRgb1" output="out" />
</shaderref>
</material>
<nodegraph name="NG_ramp_aiRampRgb1">
<ramp_rgb name="aiRampRgb1" type="color3">
<input name="type" type="string" value="v" />
<input name="position" type="floatarray" value="0, 1, 0.164871, 0.332974, 0.533405, 0.779095" />
<input name="color" type="floatarray" value="1, 0, 0, 1, 0, 1, 1, 1, 0, 0, 1, 0, 0, 1, 1, 0, 0, 1" />
<input name="interpolation" type="integerarray" value="1, 1, 1, 1, 1, 6" />
</ramp_rgb>
<output name="out" type="color3" nodename="aiRampRgb1" />
</nodegraph>
</materialx>
// Stephen Blair
// Arnold Renderer Support