Having no red coloration on the inside of the traps does not mean that your Venus flytrap is unhealthy. In fact, there are some cultivated varieties that never get any color in the traps no matter how healthy they are and how much sunlight they receive. See the article on ‘Justina Davis’.
Most growers agree that coloration in Venus flytraps comes primarily from exposure to strong light. I have also noticed a correlation with the amount of time the trap is open. Initially, when the trap opens, it usually has very little coloration. If the trap is triggered to close, it almost never develops more coloration than it had at the point when it closed.
However, if the traps are left alone, they will continue to color until they get deep red. If you’re growing your plants outside, traps also seem to color up more late in the summer and early fall. Lack of coloration doesn’t really indicate anything about the health of the plant. It doesn’t mean that it is malnourished or that it needs to be fed.
As long as the plant is getting plenty of sunlight (at least 4 hours of direct light a day), clean water (rain, distilled or reverse osmosis water) and is planted in nutrient-poor media (peat moss, sphagnum, or something along those lines), it will thrive.