It's a subject that us older collectors are getting to know more and more, but that stumps the younger generations as they get into doll collecting: color changing. What is color changing? It is when the original stock color of your doll's vinyl, eye color, outfit starts to age and then changes or fades. I'm not sure why people think that will never happen, but when you start to collect dolls that are 20-30 years old and you are still expecting their stock color to reign true it's more the exception than the rule. So let's go over some popular ones.
The Hair:
The first thing we usually see is the condition of the hair. Older played with dolls often have dry brittle hair that can be matted, ratted, or cut, have bald patches. Dolls that have been on display out of cases can often have dust particles floating on top of their hair. Re-wigging your doll almost instantly renews the condition of your doll and gives it new life. If your doll's hair is rooted, re-rooting it or converting it to a wig will do the same. More antique dolls are difficult as they may be harder to find a replacement wig and might just need cleaning or restyling.
The Make-up:
The facepaint almost assuredly goes with played with dolls (we are not talking about the person that has a doll mint in the box for 30 years that has never seen the light of day). A clear sign is that the doll looks very mono-tone in that all the color has disappeared from the face. The lips are nude, there appears to be no blush, the eyebrows might be completely faded or not there at all, etc. Easiest way to restore this is to have an artist restore or repaint the face.
The Eye Color:
There are a couple things that can happen with eyes, and wide speculation of course. Faded eye color is bound to happen after 20-30 years and if you haven't swapped out the stock eyes by then you are probably in possession of a doll that looks well.....possessed. Blue eyes will often go silver as the paint and pigment fade away. Sometimes a blue eye will start out with a darker tone and then end up a sky blue or very light blue or sometimes go completely silver with no pigment at all. Eyes that start out as a reddish brown often seen in Amber or Mahogany brown tones will shed their brown and often go red. People tend to freak out with red doll eyes and it definitely sparks the imaginations for stories and tales about possessed and evil dolls, neither that have a positive impact on doll collecting so if your doll's eyes are red, please please change your eyes. Darker browns and different shades of brown have had different effects such as turning black and then looking like the doll has hollow eye sockets, or have turned purple. Color is often made up of layers or mixtures of colors and some may fade away while others in the mix become the new prominent color. Think of it like having a stack of lenses. If the bottom one is blue and you lay a red one on top you get purple. If you add black at the back you get a darker purple. So as one color fades the second color may be revealed, etc.
What causes this? Some theorize that it has to do with faulty manufacturing, vinyl reacting to the compounds in the plastic eyes, etc. But I don't see it that way. Plastic degrades just as other materials do with age. As well fading often occurs with having sunlight hit a doll whether in a case with the sunlight hitting it or during play with a child. For me, if something is changing color or decomposing on a brand new doll - that is a manufacturing issue fault. If something is changing color or decomposing after 20-30 years of life, that's a natural course of the doll's life and simply means you need to take care of your doll by replacing the parts that are no longer good. Care and maintenance of your doll will keep extending her life. Renewing her elastic stringing (if any), fixing any rips, guarding against pilling or snagging, replacing her wig, renewing her face paint, keeping her clean, etc will keep your doll looking her best and less scary.
Antique Clothes:
If you are among the collectors who collect antique dolls then you are well aware of the ticking time clock on your doll. I once paid a premium price for a series of doll outfits that were still in their boxes from the 1930's and I was so excited. I took one outfit out of the box and it literally disintegrated in my hands. I was crushed. The hard part about collecting dolls that are really old is just that - they might literally decompose in your hands. There are more things to look out for which are yellowing and or aging spots on your doll's clothing. Handle these dolls minimally and realize that there is a shelf life on dolls and some dolls and outfits might be passed it. Here are some common things that can happen and this is across all types of dolls:
Clothing fades or discolors. Can often look like staining or blotching, but also yellowing on normally white fabrics and laces.
Shoes unglue themselves and the soles of the shoes pop off. Areas that are well used such as buckles and snaps may wear off or pop off or break and you might see signs of peeling if the shoes are vinyl or mock suede.
Clothing may disintegrate leaving large holes or compromises in the fabric.
Clothing may become stiff and rigid often called "crispy"
Dye on the clothing may bleed and can leave marks on the doll's body
Lacing, eyelets, and closures could pop off , break off from the clothing and go missing
The overall subject here is to keep a careful watch on your collection and watch for signs of aging!
This is a fascinating topic. I came to doll collecting after being a knitter and quilter. With knitting, especially wool, there's the problem of insect damage to watch for, so I knew to do that with doll outfits. With quilting there are similar problems of color transfer, fading (blues from the 70s seem to change as one quilter put it "from blues to muddy browns) and some of the silks in older crazy quilts seemed to shatter and shred.
Like you mentioned older wigs are easy to change either for matching styles and colors or some of the new wigs like two of my dolls are now sporting Candy Cane and Merry Berry.
Older dolls can also develop stinky musty odor (it can usually be cured by airing or sticking in a container with cat litter or volcanic rock bags) and material breakdown which causes an odor like vomit and can't be cured. There is also sticky doll breakdown of material which can't be cured but can be helped by washing and then applying powder. This may need to be redone multiple times, but if you want to keep that doll... I did get rid of one doll because she was terribly sticky/tacky all over and looked like she needed to shave her legs because one of our cats had rubbed against her.
I haven't tried changing eyes yet but have changed the doll's personality/character to match her eye. Some of my dolls with pinkish and purple eyes are becoming fairies and other mythical creatures. Sime people use them to create vampire characters,