No window? Go with light shades
In a bathroom without natural light, dark colors can make it feel small and closed in. Use whites, creams or very soft shades on the walls, and add color with towels, a bath mat or the vanity. Good neutral white lighting helps too.
Choose the tile and fixtures first
Tile, the sink and the toilet are the hardest things to change. Build your palette around them, then choose the wall paint. If you already have colored tile, a neutral wall keeps the bathroom from looking busy.
Moisture-resistant paint
Use washable, moisture-resistant paint, ideally with a mildew inhibitor, especially near the shower. Satin finishes clean up better than matte. Ventilate after every shower so the color lasts longer.