Is it normal for a betta's color to change? Because I bought a beautiful blue male betta on the 25th and his pelvic fins seem to have changed color, they went from a velvety blue to red with white tips. Is there any way to help retain his blue coloration?

I'm afraid not, it's all in their genes and sadly petstore bettas are a mutt mix of Veil tail genetics. In english this means they basicly can turn out way different, yours probally has a marble gene. Don't worry it's often very normal for colours to change as long as they don't get pale.