Can't say I see anything unhealthy, new leaves are often a different color. if you are talking about the very top growth those are stipule, where the leaves would branch out of. If you have had the plant for a month it's not really the stores responsibility to replace the rose. The rose is responding to your care.... which looks very good by the way. If the rose is a miniature and you haven't repotted it, you may need too as minis are often cuttings crammed into tiny pots and then fertilized so they quickly fill out... the little guy may need some more leg room....and there are a number of helpful posts in the miniature rose forum.