Q: Is artificially colored wood mulch and rubber mulch harmful to plants or
the soil?

A; I can’t say they are “harmful”, but neither does much good for the soil. An
ideal mulch breaks down slowly into humus. It feeds soil micro organisms as
it also prevents weeds and conserves moisture. Your candidates don’t do that.
Bark chips, shredded wood, shredded leaves, and other materials make a better
mulch in my opinion.