We have been using the following for the amber / white control.
It allows for separate control of the amber white led with some interesting effects.

We set up 1 fixture for rgb control, 1 for amber, 1 for white.
We used red for white control, green for amber.

Hope this helps.

<Fixture menuname="martin RGBAW" uniqueid="MAR6" excelexportid="MAR6" pixelwidth="6" pixelheight="1" output="CUSTOM" comment="Martin RGBAW" channelcount="30" channelsperpixel="5" orientation="TOPLEFT">
<channel index="1" output="RED" />
<channel index="2" output="GREEN" />
<channel index="3" output="BLUE" />
<channel index="4" />
<channel index="5" />
</Fixture>

<Fixture menuname="martin A" uniqueid="MAR7" excelexportid="MAR7" pixelwidth="6" pixelheight="1" output="CUSTOM" comment="Martin RGBAW" channelcount="30" channelsperpixel="5" orientation="TOPLEFT">
<channel index="1" />
<channel index="2" />
<channel index="3" />
<channel index="4" output="RED" />
<channel index="5" />
</Fixture>

<Fixture menuname="martin W" uniqueid="MAR8" excelexportid="MAR8" pixelwidth="6" pixelheight="1" output="CUSTOM" comment="Martin RGBAW" channelcount="30" channelsperpixel="5" orientation="TOPLEFT">
<channel index="1" />
<channel index="2" />
<channel index="3" />
<channel index="4" />
<channel index="5" output="GREEN" />
</Fixture>