How can you make the color even? I ask because last night I did a touchup on the tat in my avatar and it has darker/lighter areas. No mater how hard I tried to get the whole tat even I just couldnt do it.
It looks as though you mean black, if so, a few colour caps of black, not just one as the blood will dilute the ink, dont overwork the area as this will dilute the black, use a good quality black with good technique, are you using rounds or mags, in your case I would use a smaller round grouping, probably a 9 round so you have to move slowly and pack small areas, trying to do big areas with a big needle grouping can easily leave gaps if you have a fast hand speed, slow and small, try a piece on your leg to see what I mean.
Also putting black on top of black you can tend to work too quick as it can become deceptive, you think it has covered again but not always.
Vaseline, too much will clog the tip and you wont be able to tell as it is already black underneath and you can see the puncture marks but not necessary going in.
Also worth mentioning is the set up of the machine, i have known tattooists to use a liner machine to block fill black but you have to be careful not to damage the skin, I just use a colour packer.
Probably too much information but this is what I have learnt on my journey and asking advice of tattooist friends.
Try using mags & work the needles in small circles...do a small area at a time & don't be tempted to rush...tattoos take time !! the more you rush,the more light patches you will have because the ink isnt going in...just work methodaclly & you will get a good fill.
Mags do work better but be careful, they will tear up skin if you arent careful, they took me a while to get the hang of but once you do they are great for big areas, try a 5 or 7 mag first, you can also use the side of a mag for tight bits, the disadvantage of a big round is imagine using a 14 round, angled at 45o all 14 needles are not going in the skin at the same depth, the lower needles will enter the skin deeper than the upper needles, this may be why you are getting a patchy fill, it is always more apparent on big areas, can you picture what I am saying?
i found that the machine setup made the most diffrence with my black fill
change your springs and armature bar to this kinda setup:
0.015 front spring-0.016 rear spring. heavy (wide) armature bar.
and also mags seem to be better for getting an even tone aswell
Making the even color is not an easy task. If you want an even color tattoo then it depends upon the your tattoos making style. Sometime even color tattoos depends upon the depth of the inserting of the needle.