I just purchased an IRWIN chalk reel and I'm not sure which color chalk I should use for different applications.

Choosing the appropriate chalk if very important. Blue chalk is considered a non-permanent color. It can many times wash away but on some surfaces it can be permanent. If used indoors I always recommend white if possible. All other colors red, orange, green, yellow and black are permanent colors. It is nearly impossible to remove them. Choose your chalk color based upon permanancy, contrast to surface it's being used on.