Are the tulip bulbs and not the leaves, said my teacher. So do not waste too much time working on the details, but understand the tone of the leaves and paint accordingly. When light filters through the leaves, it gives it a pale yellow tinge while making it look translucent. When light falls on the leaves directly, it looks almost white, illuminated.
I was stupid to practice on the leaf my teacher had painted as a demonstration using the cross hatching technique, which I was familiar with using pen, but not oils. Though the method is the same, the rendering is different. Now the reference is lost and he said it was a waste of his time doing that again.
Instead I can use a rounded brush for broad strokes and details, instead of the flat one I was using which did not serve the purpose anymore. Further, if I were to follow the picture, my drawing has to be perfect. Else, if my drawing is driven by my imagination while using the picture as a reference, then it is realistic to borrow the tones from it.
Finally, using a thin brush, I will complete the finer edges of my art.
Make good art.