My eyes sometimes look dull and slightly yellowish - is it safe to use a Carboxymethylcellulose 0.5% w/v Eye Drop for this?

These lubricating drops are generally very safe and are mainly used to relieve dryness, irritation and discomfort, so using them shouldn't cause any harm. A yellowish tinge to the eyes isn't something these drops are meant to treat though, it can sometimes point to something like liver function or bilirubin levels that need looking into separately. I'd get that particular symptom checked by a doctor, while continuing the drops for comfort if your eyes feel dry in the meantime.