Ben is right. I will also add something that I found doing a forum search today. Make sure you are using the same type of batteries. Sometimes having different types of batteries can yield different results according to posts I read during my morning.
Yup, the difference is most noticeable when you vary it between regular alkaline and NiMH. This is because the regular alkaline cells each provide 1.5V, while the NiMH ones give 1.2V each. The lower supplied voltage gives you a redder color.