Iromuji is...

Iromuji is a plain kimono without black color.
Iromuji with family marks is worn as formal kimono, without family marks is worn as a casual kimono.
Iromuji ting up with a black colored obi is wearable as informal mourning. It depends on combination of Obi and gives you variety of opportunities to wear it.
This is with a gray colored shorter obi is worn for memorial service.
Wearing iromuji is preferable during a Japanese tea ceremony due to withdrawing any showy things
and avoidance to match between patterns on tea equipment and designs on the kimono. Color of choice is important, but also base pattern on kimono for occasions. It is better to NOT wear kisyou patterned Iromuji during funeral.