A changeling can alter his or her gender (and reproductive abilities) as part of using the race's minor shapechange ability.
A changeling does have a gender, as said on D&D Beyond, it just uses it as another aspect of whatever creature it's imitating. “Changelings have a fluid relationship with gender, seeing it as one characteristic to change among many others.”
Changelings are able to change their form at will without expending a spell slot or using a disguise kit, but there are limitations to their abilities. For instance, they cannot change the clothing or equipment that they are wearing. Whatever they were wearing as their previous form will carry over to their new one.
It's any humanoid. Changelings can reproduce with literally any creature with the Humanoid Type, and there is a 50/50 chance of getting a Changeling, or getting whatever the other parent was.
The short version is that a Changeling can shapeshift to a form that can produce sperm to impregnate someone, and can shapeshift to a form with a uterus and eggs to get pregnant, but a pregnant Changeling cannot shapechange to a form without a uterus without ending the pregnancy.
Most often it was thought that fairies exchanged the children. In rare cases, the very elderly of the fairy people would be exchanged in the place of a human baby, so that the old fairy could live in comfort, being coddled by its human parents.
Changeling blood is green, unlike most other animals. Changelings, despite their thinly and seemingly fragile frame, are capable of surviving otherwise fatal injuries thanks to their magical abilities.
Changelings born from two changeling parents are born in their natural form: pale, slender, with colorless eyes and white hair.
As for being tattooed, changelings can definitely be tattooed, and they can just as easily erase the tattoo with a moment's thought.
If you want it to be able to grow a tail when it changes into a tiefling or tabaxi, it is your choice. It is also your choice if you want the tail to be prehensile and able to grab and hold stuff. You can appear as a member of another humanoid race which will include things like tails, frills, gills, wings, horns, etc.
By RAW changelings revert when they DIE but not when they are unconscious (it would be hard to maintain a long term masquerade if you revert when you sleep!). I bet suggested that a piece of a Changeling — such as cut hair — WILL revert, but that this is slow and may take up to 24 hours.
Most changelings don't know they're changelings.
Though they may always feel somewhat different or removed from human life, most changelings are too young to understand who they truly are.
Changelings mature slightly faster than humans but share a similar lifespan—typically a century or less. While a changeling can transform to conceal their age, the effects of aging affect them similarly to humans.
They have large colorless eyes circled by thick black rings. Their hair color is most commonly a light shade of silver followed by platinum and blonde. In rarer cases, their hair can be pale shades of green, pink, and blue. Similar to elves, changelings lack body and facial hair.
Changelings lay their eggs directly within food sources her offspring will eat after hatching - namely stashes of meat - that the mother has procured and hidden out of sight in a secure place, such as the abandoned burrow of some small animal, a crevice in rock, or a small hollow in a tree.
Appearance. In their true form, or their form "at rest", changelings have pale skin, colorless eyes that can be completely white or completely black, and white or silver-white hair.
Changeling Reproduction
If a changeling in a female form conceives a child, she loses the ability to change her gender until after the child is born.
Changelings have an interesting relationship with gender as a concept, which is probably part of the reason a lot of the art doesn't find it necessary to distinguish the males with facial hair. As they're changelings though, they can have any kind of hair they want.
Changelings typically have large eyes with narrow pupils, ranging in color from white, to yellow, to black. Hair colors are similar: Light grey, platinum blonde, shades of silver, all the way to the darkest hair color being a light brown.
Changelings don't inherently recognize each other. If they don't want to be recognized then when they do something suspicious it would be an insight check against the changelings deception, which the changeling has advantage on. A changeling could have expertise in insight from divergent persona.
17. As a playable race, changelings have the following abilities: Shapechanger: Using an action they can change their appearance and voice at will, even adapting their size between small and medium in some cases.
Changelings were fairies who had been left in place of a human child or baby who had been stolen by the fairies. The child was taken for one of three reasons; to act as a servant, for the fairies to receive the love of a human child, or for malice/revenge.
Some types of octopus, squid, and crustaceans have blue blood. Their blood contains a high concentration of copper. When copper mixes with oxygen, it gives their blood its blue color. Blue and red are not the only possible colors of blood — some animals bleed green.
Oxygenated (arterial) blood is bright red, while dexoygenated (venous) blood is dark reddish-purple. The difference is color results from the electronic state of the iron ion (ferrous vs ferric), which in turn influences the π → π* and n → π* electronic transitions of porphyrin and hence its optical characteristics.
Blue Blood is a deep, neutral, Caribbean aqua with a turquoise undertone.