Hi, welcome! He's a little cutie! The tail nip is common, unfortunately, we know you didn't do it, breeders often keep too many babies together for too long and this happens, along with toes being nipped completely off. Not an issue at all for him though.
As far as his dark color, I agree that he just looks cold. At 3 weeks home with you in his new enclosure most of his relocation stress should be gone, and it sounds like you've got a good UVB tube light, it's an 18" long tube, 10.0, but which brand is it if I can ask? Not all are created equal. And the distance of the UVB tube should be relative to his basking spot and where he will be sitting,that's what you need to worry about, not how far from the bottom of the enclosure it is. Depending on what brand UVB tube you have and what strength it is, I.e. is it a T8 or a T5, this determines how far it should be from his main basking spot. T8 tubes usually need to be within 8" of his basking spot at a maximum distance, T5 UVB tubes usually need to be at a distance of 11" at a minimum from his basking spot. Is the UVB tube mounted inside his enclosure, or is it sitting on top of a lid, like a glass or metal mesh lid?
UVB light cannot penetrate glass at all, so you can never set a
UVB light on top of a glass lid, and you cannot have any type of clear glass or plastic cover on the UVB tube fixture covering the tube. A lot of tube fixtures come with a clear, plastic cover over the tube for protection from the tube, but they emit little to no heat, and the plastic cover can block up to 50% of the
UVB light. A mesh lid will also block up to 50% of the
UVB light, and any type of glass blocks 100% of the
UVB light. So please never think that he's getting UVB through a window, he's actually getting zero. Remove any clear plastic cover from your UVB fixture. And if it's a T8 10.0 UVB tube then you want it mounted inside the tank, as a T8 tube is not strong enough to penetrate the mesh lids, there needs to be NOTHING BETWEEN your beardie and the UVB tube.. If your UVB tube is a T5 then you can keep it on top of a mesh lid as long as it stays within 11" of your beardie on his basking spot.
And as far as the two basking lights you have over him, they should both be bright white, no colors at all like blue, yellow, red, etc. Your basking light needs to sit right alongside your UVB tube over top of his basking spot so that he gets both the basking light/heat and the UVB unobstructed at the same time, mimicking natural sunlight. Do not have the UVB tube centered over the enclosure and the basking light over the basking spot, I see that a lot and it's not giving the beardie both lights at the same time. Your lights need to be on for at least 12-14 hours every day, and no lights at all on at night. As long as your nighttime temp stays above 65 degrees he needs no nighttime heat source, the temps must drop just like it does outside.
In my opinion, take this how you want to, temperature guns are not as good for measuring reptile enclosure temps as the digital thermometers that have probes on a wire. You can actually set the probe right on his basking spot, exactly where he will be sitting, allow the probe to sit for 20-30 minutes, then read the temperature, and it will be a very accurate basking spot temperature. Then you can move the probe down to the floor of the enclosure on both the Hot Side and the Cool Side, let the probe sit for 20-30 minutes each time you move it to a different location, then read the temps. They will be extremely accurate and will be taken right from where your beardie will be sitting and laying. They only cost $9-$10 at Petco or PetSmart, and are well worth it. I have 2 of these on each of my 3 enclosures, on on the hot side/basking spot and one on the Cool Side.
For a baby the temps need to be within the correct temperatures in order for him to not only heat up, but to be able to properly digest his food and absorb his Calcium and other nutrients. This is another reason your UVB tube needs to be right alongside the basking spot, because after he eats he's going to go directly to his basking spot and lay, because he knows he needs to bask under both the UVB and the heat so that he'll digest his food.
Basking Spot: between 105-110 degrees Fahrenheit maximum for a baby, between 98-103 for an adult over a year old
Hot Side: between 88-93 maximum
Cool Side: between 75-80 maximum (very important you have this gradient, he needs a COOL SIDE to stay below 80 degrees so that he can cool down when he wants too. Cool sides are very often too hot, people don't think it's important, but it is, almost as important as the basking spot being between 105-110 so he can be warm enough and digest... FYI, anything over 110 degrees Fahrenheit on the basking spot and you're cooking him.
Your temps are definitely too low, you should see him spending most of his time on his basking spot, especially after he eats. So if he's not it means either his basking spot is too cold or he's not getting adequate UVB and heat at his basking spot. The temp gun you're using is either inaccurate or you're not using it correctly, the digital probe thermometers are pretty fool-proof.