I'm red-green colorblind. It's tough for me to discern the difference between the activated and non-activated color on the post voting buttons. Not impossible, but difficult enough where I have to stare for a few seconds and maybe squint a bit.
I've examined the colors on a Samsung Galaxy S3 and an HTC One E8, and played with the brightness settings on both. It remains difficult in all cases.
Increasing the saturation and/or luinosity value of the red activated color would help. In this case, I'd like to suggest also changing the color entirely. A blue or purple would fit the app's color scheme better than red in my opinion, though I'm not a professional or particularly skilled artist or designer. But R-G colorblindness is the most common variety, so shifting to a blue hue would theoretically reduce the number of negatively impacted users.