Global colors changes apply to the entire section within an email. This can affect multiple blocks within the same area. For example, "Body Background" will apply the changes to the main area of the email. This could affect all blocks within the area (provided the blocks have not been assigned background colors). Block changes apply only to the block chosen.
Block changes, however, will be given preference over global changes. For example, if the global color for the footer is black, but the footer block is assigned a background of white, the email will appear with the white background instead of the black one.