Environment

Windows build number: 10.0.18362.0
Windows Terminal version (if applicable): 0.7.3291.0

Steps to reproduce

Allegedly already fixed in #313, but doesn't work in v0.7:

echo -ne '\e]4;15;rgb:ab/cd/ef\e\\'
echo -ne '\e]4;16;rgb:ab/cd/ef\e\\'

Expected behavior

Set palette index 15 and 16 to the given color.

Actual behavior

Color index 15 is set to the given value (works for 0..15), index 16 remains unchanged (doesn't work for 16..255).