I’ve seen lots of white toothpaste. Lots of toothpaste in greens or blues. Red Toothpaste (currently using cinnamon-flavored, in fact). Toothpaste striped in blue or green or red or some combination. There are even black toothpastes (promoting the benefits of charcoal, from what I’ve seen). But I don’t ever recall seeing yellow toothpaste.
… or, come to think of it, purple. But mostly I’m wondering, why no yellow? It’s a bright, primary color.
Because plaque is yellowish and staining from other causes (e.g. smoking) is also yellow.
Yes, obviously there’s no reason why a yellow coloured toothpaste couldn’t be just as effective as any other color. But that psychological thing of not wanting to buy something that is the color we *don’t* want our teeth to be, makes it a bad choice for a brand to try.
ETA: There are of course some examples, even if they aren’t the most common things on the shelves.
There are a number of lemon-themed toothpastes. It’s hard to find a good image of the paste itself, but several seem to be either soft yellow or have yellow bits in them.
I think turmeric toothpaste might be yellow.
Yes, red is associated with a cinnamony flavor. White, green, and blue (at least, the shades commonly found in toothpaste) are common colors for something that tastes minty. What toothpaste-approrpiate taste is associated with yellow? (Or did @Acsenray already answer that?)
Well, uh, historically there is this
Lemon
Why is there no yellow toothpaste?
For the same reason there are no yellow diapers.
Or yellow toilet bowl sanitizers.
Once we get yellow toothpaste, the next thing you know, there’ll be brown toothpaste. 
I don’t know why we don’t have banana-flavored yellow toothpaste, but yellow is probably the natural state of teeth color in human adults…
It is perfectly natural to have yellow teeth and this is not necessarily a sign of poor oral health. Even with a healthy lifestyle, factors outside of our control can affect the appearance of our teeth such as our genes and the thickness of our tooth enamel. Our teeth often appear more yellow over time as part of the aging process. Dr. Hindocha told Live Science: “as we get older, our enamel thins or breaks down which means that the inner part of the tooth shows through more, making the teeth appear more yellow or creamier. Additionally, staining from certain foods and drinks, including tea and coffee, can also contribute towards the yellowing of teeth.” Yellow teeth are not necessarily a sign of poor oral health, though in some cases it may signal that the enamel has eroded and that the teeth could be weak and prone to chipping.
Yellow is the color of a lot of body fluids, secretions, and excretions (normal or abnormal) that are considered “yucky”. So I imagine there would be some market resistance to a dentifrice in a “yucky” color.
Yellow food coloring dyes are some of the most staining dyes out there. Ever try to get a mustard/turmeric stain out of clothes? Why on earth would you use it on teeth?
I bet that Pepsodent slogan from way back in the 60s has a lot to do with it.
Appears to be a UK thing, gawdhelpus. So it exists, but it scarcely weakens the argument.
Lush: “Fresh handmade cosmetics” and so forth.
j
I think the answer can be summed up as “There is no yellow toothpaste for the same reason that there is yellow snow.”
In the 60’s, Japanese manufacturer Sunstar had banana and strawberry flavored toothpaste. Only tried the banana and I’m sure it was yellow. https://order.mandarake.co.jp/order/detailPage/item?itemCode=1090540307&lang=en
The also make strawberry and grape toothpaste: https://www.japan-spread.com/category/select/pid/3196
Red toothpaste can result in “pink in the sink”, similar to the effect of minor gum bleeding. No one wants bleeding gums, yet we have red toothpaste. So… I’m not buying that as the total reason. Likewise, no one wants black teeth, yet we have black toothpaste. Again, not sure “yellow toothpaste looks like plaque” is the explanation.
I don’t recall ever encountering that (and at one point I used to stock stuff in the “health and beauty department”, which includes the toothpaste aisle).
I suspect that it might simply be that, as several have alluded to, yellow toothpaste would just look gross.
I agree and think it is that simple. Plus I’ll listen to the marketing guy. 
I would say it is the primary reason.
“Pink in the sink” is not the same thing as the actual appearance of the teeth. Regardless, I don’t think red toothpaste is very common either.
Pink, or striped toothpaste (with one red stripe)? Sure. But actual red toothpaste? I haven’t personally seen it.
Charcoal toothpaste is an interesting example. If you google it, there are countless FAQs where one of the first points addressed is the counter-intuitive nature of using something black to whiten the teeth. It took selling the concept for it to catch on.
I think prior to the charcoal fad, a brand making an ordinary flouride toothpaste, but showing the paste as being black on the packaging, would have struggled. In itself, the color would be a detriment to sales.