Blue - There's constantly competition within the Taverly Dungeon (the which you're at OSRS GP the moment talking about fishing for eels from). However the resource dungeon in it has between 5 and 6 Blue Dragon spawns and isn't overly crowded. I personally kill blue dragons here, but it is required to have a minimum of 60 dungeoneering.
Red - They're found in the Brimhaven dungeon , or Red Dragon Isle in the deep wilderness. Both are very far away from banks, and not worth the effort. Blacks without your stats and considering you can only kill 5 I wouldn't recommend killing them too. Please update the original post to include your full stats next time around to keep an eye in this discussion. Thanks!
I'm not sure what to get in the subsequent DG reward. I'm considering getting the Rigour prayer or the Chaotic crossbow. Let's say that you have 100 hits , with 150 damage per successful shot. It is possible to estimate that 20 of the hits will be zeros and give you an approximate rate of 80 percent.
Now we can factor in an increase in accuracy for the Crossbow the accuracy will increase to around 87% and each of the extra seven shots you manage will also average 150 damage of course. This means that your average damage has increased to 130.5 as opposed to be 120 (this factors in the 0s). We can now compare this to the same increase of 5% in damage:
Your accuracy rate will remain at the level of 80%. As each of buy RS gold the hit will get increased by 5percent, the overall damage per successful hit goes up to 157.5. We will now add your 20 zeros you hit for every 100 shots. Your new overall damage per shot will be 126. Therefore, you'll see 20 percent more damage, rather than a 5% increase however I'm removing the damage that eagle eye already offers.