Hi, welcome to the forum. This is totally normal, don't worry, most dragons do have a tail that is darker towards it's end than the upper portion. Totally normal. You only need to worry if his tail turn pitch black, like black black, and this is usually accompanied by the jet black portion shriveling up and even falling off. It's also typically due to some type of open wound or a band of "retained" shed that goes all the way around the tail and is cutting off the circulation. This is what is referred to as "Tail Rot", which is nothing more than an infection that is caused by either an open wound/bite that becomes infected or the circulation being cut off and this caused the tissue below it to die and turn black (necrosis or necrotic tissue), and if it's not treated the infection will continue to travel up the tail, killing all of the healthy tissue along it's way, until it reaches the bloodstream and then causes a septic infection. Trust me, you'll know the difference if this happens, the tail literally dies. But what your dragon's tail is showing is nothing more than a darker shade towards the end of the tail than the top of the tail, which is completely normal.
I will give you a bit of advice or an FYI though: #1.) Most babies and juveniles under a year old will eat little if any greens/veggies at all, and though they should be offered fresh, healthy greens/veggies every single day just so they become used to them and will eventually start picking at them, if they do eat them you should consider it nothing more than a bonus and not a part of their daily diet at all. Bearded dragons continue to grow up until they are 2 years old, but 85% or more of their growth and development happens during their first year of life, so this is why they don't eat many if any greens/veggies when they are under a year old, and nor should you try to do anything to force them to. They need tons and tons of live insects every single day up until they are a year old because without them during their first year of life they won't grow or develop properly,. their bone density will be weak, and they will end up being stunted.
It's awesome that you give him at least 2 live insect feeding sessions a day (3 is better up to 8 months, then go down to 2 per day), but 5 small crickets isn't near what he should be eating (assuming the crickets you're feeding him are smaller than the space between his eyes, as they must be so he doesn't choke and so his body can properly digest them). Each feeding session should last between 10-15 minutes, and he should be allowed to eat as many live insects as he wants during each session. Most people using crickets throw in -3 at a time, let them eat them, then throw in another 2-3, etc., until he walk away and goes to bask or the 15 minutes is up, whichever comes first. Please never leave any crickets in his tank after he stops eating them, get them all out, as they hide under and behind things, and they are well known to bit dragons while they sleep, which can cause horrible infections and they can even bite their eyeballs. So get them out after he's done eating.
If he's only eating 5 small crickets twice a day, this is most likely due to either inadequate
UVB light, or improper temperature zones, or both. What
UVB light are you using, and what are his Basking Spot Surface Temperature, his Hot Side Ambient (air) Temperature, and his Cool Side Ambient (air) Temperature? This needs remedied ASAP, as he should be eating more like 20+ live size small crickets every feeding session, 2-3 times a day.
Stress marks?
