I carry two of these Camelbak bottles (above) in my Prius for my travels. One I usually have water in it and the other I often make herbal tea. The color does hide the buildup from making my herbal tea, by just letting the tea to brew at car temperature. The other is smoke gray and also not easy to see buildup inside.
I found this problem last year when I started making my own herbal tea, I noticed the nipple that you bite on the bottle to drink had black in it. After a detailed investigation, I was making a face at what I was drinking though, and I went out and bought a straw brush shown below. Once in awhile when I washed dishes I would disassemble the parts on the bottles and wash them. The brush is long but did get into the nipple to clean by twisting it to get into the cracks inside. Cleaning the bottle meant putting a wash cloth or sponge inside with soapy water and shaking the bottle. Not the best results, but it was acceptable.
Note: the above bottles are the ones I use with the SteriPen to sterilize the water (see separate post). They arte 750 ml size and the SteriPen only has two settings, 1 liter and 1/2 liter, so I set the SteriPen on 1 liter for sterilization.
I think I bought the straw brush (above) at Bed Bath and Beyond.
My 2 Liter CamelBak also needed cleaning, but since it was just water there was no grime. The straw brush worked well on the tube but the bag I just flushed out and shook with some soap in in the best I can.
This year, I again went to Bed Bath Beyond and I found a bottle cleaning kit with 3 brushes. The long large brush worked great on the inside of the bottles and the CamelBak bladder bag. The short brush works great on the CamelBak bottle straws as they are short. It also worked good, not great, on the nipples. The nipples still require angling the brush tip into the inside cracks to get it clean. The short handled circular brush works good in the inside of the cap on the CamelBak bottles and the threads on the bottle.
The above are now items that I carry with me for cleaning my bottles and bladder bag. The 3 bottle brushes fit in one of my cooking bags that fit under my sleeping area and the long straw brush gets slid under the plywood floor (void space) I built for the cooking bags to slide in on.
For small vehicle dwelling, like my Prius Campervan, these brushes have become important to me in keeping my drinking containers clean.