During visualization, if some parts come up clear, some parts not so clear, we need to look at those that are not so clear again and again. If the image to be visualized and that which appears are different --for example, we visualize a sitting buddha but a standing buddha appears instead when we close our eyes; we visualize a tiny image but a huge one appears; we visualize a colorful image but a black and white version appears, and so on—we still need to look at the deity image repeatedly and steadily. With practice, this problem can be solved.

~Depicted from GATEWAY TO VAJRAYANA PATH - The Generation Stage