A single Yellow Rose can be given to your partner as an expression of happiness and love. It means you are happy with your love and relationship.
However, the Yellow Rose also has a dark side to it. During Victorian times a single Yellow Rose was used to express jealousy and infidelity. So sending a Yellow Rose tells your partner that you are jealous, or suspect them of cheating on you. So you better send a note with the Yellow Rose, so the person receiving it knows what you mean.