Ah yes it's a separate question when we have someone who does not yet suffer from dementia but is at risk and who can plausibly take some actions to reduce the risk / delay the onset of dementia. I am definitely no expert on this but I would assume promoting better physical and psychological health reduces dementia risk alongside other bad side-effects of aging. The psychological part is key. I think if you know someone at risk of dementia, making sure they have a supportive family and/or friends and they don't feel lonely goes a long way to promoting better psychological health and reducing dementia risk, even if, for example, their diet and exercise regime is not optimal.
I have heard this recommended by others but never looked into the research. Anecdotally I have found that very elderly patients/people with dementia react positively to colorful things. However, a casual googling of this revealed that "colorful stuff for dementia patients" is more of a "folk wisdom" being passed around as opposed to something with a rigorous backing (there are articles about the phenomenon like this: https://www.enablingenvironments.com.au/colour-perception-and-contrast.html). My theory of why it might be good is a) more sensory stimulation which keeps the brain active, especially in more severe cases b) helps them distinguish and remember the locations of different items more easily c) association with positive emotions (bright -> happy) d) as eyesight declines, brighter colors are required to elicit the same amount of attention / visual stimulation
Dementia patients become increasingly dependent on their caregivers, and as a caregiver, it isn't a great strategy to rely on giving the dementia patient advice and trying to convince them of something. Instead, you can model your role as trying to maximize the happiness of the patient's remaining lifespan, primarily through interventions that don't rely on the patient being agent-y. Some things that I have done for relatives with dementia that seem net positive:
Ultimately, communicating with and helping a dementia patient is extremely far removed from dealing with a rationalist. Instead, communication is primarily based on emotion and senses. In my experience, looking after dementia patients well, by providing the company, emotional support, and physical things that they need, slows down the progression of the disease as much as possible. Without this, dementia can progress extremely quickly, however in the presence of effective help, I believe dementia patients can experience net-positive lives for many years, as well as enrich the lives of their friends and relatives.
I often use the following way of explaining time estimates. "If I was to do thing X and not stumble across any issues I haven't accounted for / unknown unknowns / circumstances don't change, it will take me about 2 weeks" followed by "in my experience, most of the time issues do come up, therefore a more conservative estimate is on the order of magnitude of months, not weeks." This way I can calibrate with others who are estimating "how long does this take if everything goes completely smoothly" while also bringing up how unrealistic this is.