The following correctly renders a text shadow in your demo app:

This is a <span tts:textShadow="10% 10% 0% #11223344">test</span>.

However, if the color is specified as follows:

This is a <span tts:textShadow="10% 10% 0% rgba(233,30,99,255)">test</span>.

the test app shows the following:

error: Cannot parse styling attribute http://www.w3.org/ns/ttml#styling textShadow --> 10% 10% 0% rgba(233,30,99,255)