Color has never been my expertise, so I hesitate to offer advice on much of the colored pieces we see posted here. However, I'll give it a go. Your linework and proportions look good. The flowers and the butterfly are very realistic. I bet if I pulled out some of my college text books on botany and entomology I could identify the plant and the butterfly because of your fine work. I also like the hazy edge to the background.
Being a black and white sort of fellow (charcoal and paper), I suggest that some more contrast should be present. I suspect that the leaves and the flower petals have lighter and darker areas on them if they are scrutinized closely in your reference picture. Behind and below the flower a darker value could be added to give it the needed shadows. Inside the flower by the pistil darker shades of colored pencil could be used to hint at the shadows that should be present within the heart of the flower.
How do you do this with colored pencil? I'm not sure. ((That's why I shrink back from painting and the like.) But if you put more contrast on the subject, it will "pop out" for the viewer. Find the highlights and the shadows of your subject before you draw. Carefully render these details and the finished picture will look even more realistic than what you have accomplished here.
--JT
Let all you do be done with kindness and love
