Once you have detected the particles by thresholding the image, they should already be red. But if you want to change the color, you can use the IMAQ UserLookup VI (found in the Vision>> Image Processing>> Proceessing subpalette) to create your own lookup table for the binary image. Since it is a binary image, you should only need to input a 1-D array with 2 values: the first being the background color and the second being the fill color of the particles.
Hope this helps-
Julie