unfortunately no more user of ImageJ… so I am not spot-on, also especially with your request for measuring intensity of Bromophenol stain for protein demonstration.
Perhaps you might gain more specific information on visiting:
KURU 2014_‘Optimization and enhancement of H&E stained microscopical images by applying bilinear interpolation method on lab color mode‘
https://__www.ncbi.nlm.nih.gov/pmc/articles/PMC3923735/ (after copy and paste - before pressing the browser button - delete the two underline signs in between https:// AND www.ncbi....)
For simple intensity measurements you can try out a free trial of our Phoretix Toolbox software here: https://totallab.com/products/phoretix-toolbox/
Simply draw around the stained areas you wish to analyse and the volume measurement will give you the intensity of that shape amongst other measurements.