Brian,
Thanks for the reply.
I used your code sample originally when writing my code.
I didn't use 'Paint - Enamel Glossy (Yellow)'
Instead I used 'Powder Coat (Blue)', I didn't want glossy.
It worked fine for me.
However, when I sent my add-in to the app store for publication, their test said it was a bad name.
I did some testing and found that only downloaded appearances would work.
Not knowing what the end user may have downloaded, I want to know if it is possible to download programmatically.
This is my original code taken from your sample.
plateTCPext.name = 'Plate TCP'
plateTCPbody = plateTCPext.faces[0].body
plateTCPbody.name = 'Plate TCP'
fusionMaterials = _app.materialLibraries.itemByName('Fusion 360 Appearance Library')
appear = fusionMaterials.appearances.itemByName('Powder Coat (Blue)')
yellowColor = design.appearances.addByCopy(appear, 'YellowColor')
colorProp = adsk.core.ColorProperty.cast(yellowColor.appearanceProperties.itemByName('Color'))
colorProp.value = adsk.core.Color.create(204, 204, 0, 0)
plateTCPbody.appearance = yellowColor
Brad Bylls