Final answer:
The correct transition for the sentence is 'In fact,' emphasizing the significance of Van Gogh's use of color in his paintings and techniques like impasto and hatching.
Step-by-step explanation:
The transition that best fits the blank is C. In fact. The sentence discusses the importance of color in Van Gogh's work, emphasizing it as his primary mode of expression. This is not a contrast or a sequence of events, but rather an assertion that supports the previous statement about Van Gogh's use of color, such as in the description of how he applied the impasto technique with thick and swift strokes, or how his fabulous hatching technique evoked the textures it depicted, appreciable in the varied tones of his paintings.