The town is bathed in Christmas spirit… Christmas Eve is just around the corner.
Literal
Town [topic-は] Christmas-mood one-color... soon-already Eve is.
クリスマスムード一色 is a fixed atmospheric description: ムード (from 'mood') plus 一色 ('one color, all one color'), meaning 'completely (dominant mood).' The pattern [noun]一色 is productive: 桜一色 ('all cherry blossoms'), 秋一色 ('completely autumnal'). もうじき ('almost, soon, any minute now') is a casual near-time adverb. イブ is short for クリスマスイブ ('Christmas Eve') — a major celebration in Japan despite the non-Christian context.