1. An image measurement apparatus comprising: an image acquiring unit configured to acquire a sample image that is an image obtained by capturing an image of a sample; a standard color space converting unit configured to convert the sample image into a standard sample image that is an image in a standard color space; a color distribution normalizing unit configured to convert the standard sample image into a normalized sample image by converting pixel values of pixels in the standard sample image to cause a color distribution vector indicating a color distribution of the standard sample image to match a standard color distribution vector indicating a standard color distribution and predetermined based on a plurality of reference standard sample images; and a positive rate acquiring unit configured to detect positive pixels by performing threshold processing on pixel values of pixels in the normalized sample image, and acquire a positive rate indicating a proportion of the detected positive pixels, wherein the plurality of reference standard sample images are images in the standard color space of a plurality of reference sample images obtained by capturing images of a plurality of samples prepared from the same reference specimen by using a plurality of sample preparation methods having different characteristics.