This is an issue with using PNGs. Read about it here: The Sad Story of PNG Gamma “Correction”
I assume you are using a PNG for the arrow and the rest of the bar uses CSS? But you need the arrow to be PNG for alpha transparency on the black bit? Can you instead make the arrow a .gif with full transparency and stretch the black bar all the way across underneath the .gif?
Or you can use a 1px png of the colour, and use that to fill the rest of the pink bar instead of CSS. This will make it match the arrow at all times, but it means that that particular bar+arrow will be a different shade of pink from other areas where it was intended to be the same.