1. A white-balance method for use in a multi-exposure imaging system having an image capturing unit, the method comprising: utilizing the image capturing unit to simultaneously capture a first image and a second image of a scene with a first exposure value and a second exposure value, respectively, wherein the second exposure value is lower than the first exposure value, and the first exposure value and the second exposure value comprise individual exposure time and exposure gain; calibrating the second exposure value, so that an average brightness value of pixels in the second image is equal to a specific ratio of a maximum sensing value of the image capturing unit; performing light source detection on the second image to obtain light source information and a light source color ratio corresponding to the light source information, wherein the light source color ratio of red, green, and blue colors is a:b:c; and performing a color gain process on the first image according to the light source color ratio to generate an output image, wherein red, green, and blue colors of each pixel in the output image in the color gain process are respectively defined by the following formulas: R ′ = p × R + ( 1 - p ) × b a ⁢ R ; G ′ = p × G + ( 1 - p ) × b b ⁢ G ; B ′ = p × B + ( 1 - p ) × b c ⁢ B ; where R, G, and B denote pixel values of red, green, and blue colors in the first image; R′, G′ and B′ denote pixel values of red, green, and blue colors in the output image; and p denotes a weighting factor between 0 and 1.