All rainbowfish start out with no color. They develop their color as they age, and when that happens depends on the type of rainbowfish. They also age rather slowly, so it can take awhile before they acheive their full color. The ones you find in the LFS will usually have a bit of color and be around 3-4 months old. Depends on the LFS and their supplier.
Now, if you go into a store and ask for a New Guinea Rainbowfish, you are going to be disapointed -- those fish are not red. Body-wise, they don't even look alike.
The red one you are talking about is
Glossolepis incisus or Salmon Red Rainbowfish. It's also called the Irian or Irian Red Rainbowfish. This fish originated in Irian Jaya, thus the name.
http://members.optushome.com.au/chelmon/Incisus.htm
The New Guinea Rainbowfish is
Melanotaenia affinis. It is a totally different color and originated in Papua New Guinea.
http://members.optushome.com.au/chelmon/Affinis.htm
Make note of the the scientific and official common names of the fish you want so that you don't get the wrong ones. It happens all the time.
I hope you have a large tank because
G. incisus grows to 6" and needs to be in schools of at least six, more is better. A 45-65g tank is the minimum size I would put these guys in.
Roan