It is difficult to judge the value of local colours when a mid-tone is placed among other mid-tones. Blurring the image by looking through half closed eyes or using black and white photo of the object seems to solve the problem. This way you can establish the outline (shape) of the object (focal point) distinguishing it from the background.
I find that I'd rather like to look for the darkest shapes and then add a mid-tone, then leaving lighter tones as the white of the paper. Keeping it simple as possible helps to not get overwhelmed by the details.
Coffee Painting will change you. Connecting with the earthly organic medium and the pleasant aroma only improves your ability to express your artistic vision. Although monotone in theory, the many shades of coffee range in colour from golden to chocolate to sepia to burnt umber - like colours. Playing with the rich, warm tonal values of coffee also shows you the importance of light against dark and sometimes the darks of the shadows have an interesting depth unlike watercolour. Altogether you will find harmony and cohesiveness in your final production.