pdf – Why does the preview make colors blend in these pdf images pdflatex?

I made the following image in PDF format (a figure for a scientific paper):

colored grid with different colors

On my Linux machine, I opened this image in evince and okular and it seems to appear here very well. It also appears this way in Google Drive.

However, when I open it with Preview on my Macbook, it looks like this:color grid with mixed colors

Why do colors blend in the Preview and is there a way to prevent this from happening? (I will have to send this figure as part of a job for a printer to print and I would like to ensure that the figure remains "whole" for printing).