I have chosen this fairly simple photograph to show you how to deal with tonal values and manipulate them if necessary. I did the sketches using a 4B pencil with a blunt tip to prevent meticulous detailing.
First, draw the large contour shape and the shoreline. Next, I drew some light “enrichment shapes” in the “dark” example, in which I did not focus on reality so much as on creating a pleasing light pattern across the image from left to right. After that I indicated the dark silhouette of the city using vertical shading, and the sky and water using lighter shading. You can see that I limited myself to three tonal values: the lightest value to suggest light hitting the buildings and to add liveliness to an otherwise dark, dull silhouette. Also note that the dark value of the bridge has been absorbed into the whole of the subject (in the second example, the bridge gets center stage). Finally I want to point out that the minarets and the added suggestion of a boat serve as connecting elements between the main subject and the sky and water respectively. The three elements making up the image – sky, middle ground, and water – would be too separate otherwise.
In the second example, the same composition is translated into a background in middle value, light value for sky and water, while the suggestion of the bridge is the focal point here, which gets the maximum value contrast. A few vertical connecting elements have been added here too, not because it looks nice, but because the subject demands it (the dividing line between bridge and water would otherwise be too harsh). Now, to develop such an example in watercolors based on these sketches is simply delicious. But we are not nearly there yet. You will have to know a lot more about tonal values to produce decent work, not just once by accident, but again and again, because you have mastered the basics.