Final answer:
The confusion between Paul Cézanne and Vincent van Gogh in the question notwithstanding, a serious self-portrait by Paul Cézanne can be described as having a serious tone based on sober colors and methodical brushstrokes, unlike Vincent van Gogh's more vivid and expressive self-portraits.
Step-by-step explanation:
Although the question appears to conflate the artist Paul Cézanne with Vincent van Gogh, a careful reading indicates that the student is referring to self-portraiture in the context of seriousness. To address this, one can consider how Paul Cézanne's self-portraits are often characterized by a serious tone, which can be deduced from his use of solid, sober colors, methodical brushstrokes, and the direct, introspective gaze found in several of his self-portrait paintings. On the other hand, Vincent van Gogh's Self-Portrait with a Straw Hat emits a brighter, more vividly expressive mood. This can be observed in van Gogh's summer-style clothing, the juxtaposition of colors, and the experimental nature of his brushstrokes, showcasing his post-impressionist influences and a more dynamic exploration of self-representation.