TTML 1.0 (Editorial)
TTML 1.0 Test Suite
TTML IMSC 1.0
TTML IMSC 2.0
TTML Simple Delivery Profile for Closed Captions (US)
Frans de Jong
Loretta Guarino Reid
Philippe Le Hégaret
tts:color (and other properties that use colors) don't define what a presentation processor should do if a specific color isn't supported; as such, this means that the default semantics of XSL-FO or CSS2 hold;
we should consider defining something specific or add a note that references the interpretation of unsupported colors in XSL-FO or CSS2 as appropriate
Add notes (no markup allowed, URIs get automatically hyperlinked):
I see the following language in 8.3.2:
"If a presentation processor does not support a specific, valid color or alpha value, then it must interpret it as being equal to the closest supported value."
This may be adequate to close this issue.
Added further note in 8.3.5:
"In this context, the phrase closest supported value means the value for which the Euclidean distance between the computed color or alpha and the supported color or alpha in the RGB color space is minimized."
I believe this address this issue. See https://dvcs.w3.org/hg/ttml/rev/24ed1e6adcdd.