First of all Congratulations for your attempt JoyCF! Well done
Because l was watching your stages in painting this wonderful subject ,and l myself am a passionate in copying other painters works,l found useful for you to give you some advices that could help you in your goal (in painting)
The underpainting you have done is very helpful in obtaining halftones in flesh color,and other areas ,and it was developed in a period of approximately 300 years (from the beginning of 15th to 18th century ).
The painting you've copied is from 19th century (France Academy).The general painting method in that period was,for the underpainting,the wash-in method ,that consists in painting all the form with the local color,but very thin,on a toned canvas (the color was chosen depending on the luminosity of the finished painting,but in general an earth color).Then followed scumbles ,glazes,etc.
Because you have used dead layer ,the upper layers ,being very thin applied ,lost their intensity ,and the color became grayish.
My advice for you is ,if l have your permission ,to paint another layer,thicker ,and richer in color ,as in the original. Just go all over the painting with another layer
And to help you ,l made some research for some studies of Bougureau ;here's the underpainting,and l've put side by side your painting and Bougureau's .Apologies for my boldness,but we are here to learn from each other,and to develop our skills