Why is the screen so dull on Garmin watches, including the Fenix 6. In bright daylight, it's great. But otherwise and especially indoors, it's dull. But why? I have a couple of old Casios, and they are significantly brighter, as the linked photo sh

The old Casio has no colors and a very low resolution...

Its a power saving feature, as I understand it the individual pixels in the screen are set once into memory, and then require no additional continuous power unless their state needs to change.  The resulting look from this technology is as you describe, its great in bright light outdoors, but is a little worse in brightness and contrast indoors unless you use the backlight.

Black and white LCD screens have higher contrast and usually lower resolution. The Fenix has colors and a much higher resolution. The screens that provide those are lower contrast. 

An unfortunate trade-off in that case. 

I love these screens. Good to see at daytime, don't disturb in darkness and easy to switch on.

For me, they are the closest to classic wrist watches in terms of everyday behaviour.

My concern was not with having to use the backlight at nite but their dullness indoors during the day, forcing my aging eyes to use the backlight then, too. 

yes, it is normal, this is the way the screen works.

it is a transflective memory-in-pixel technology.

• -- not so bright indoor - the AMOLED screen consumes a lot (a smartwatch has to be recharged almost everyday). yes is bright.
• ++ the garmin screen consumes power only when a change is to be made in the display.
• ++ once set does not consume power - therefore you have 24 days (it is an example) of power
• ++ in the bright light is fantastic visible, as the light is reflected differently by the pixels.
• ++ keep in mind that is an always on screen!

indoor yes, you will feel it is a little bit darker, but set the backlight triggered by hand movement (at least in the period you are awake)...or a simple upper left button push will show you all you need.

a 10-20% backlight level is enough for most, I use 30% because of my bad eyes...

I like it exactly because of this technology!

enjoy it, it is a fantastic watch!!!!

just me,

robert

I want to add to what others, esp. marcelvos1987, wrote (B/W displays have higher contrasts) that you have to look at the Garmin Instinct to see that´s true:

The Instinct´s display seems to be way more crisp despite having a much lower resolution. The lower contrast, resulting in the "dull" impression, is the trade off to the screen with 64 colors.

The Casio is not amoled

This might be an obvious point, but this is a GPS sports watch with massive battery life. It's primary purpose is for tracking OUTDOOR activities. As such, the design choices have been made so that the screen is easy to read in full sunlight without battery-sucking backlight. Ultrarunners and Ironmen/women of the world rejoice. 

Compare this with your phone with the backlight/auto brightness turned off. Utterly unreadable in direct sunlight outdoors. 

My suggestion would be to adjust your Backlight settings for Not During Activity to Gesture. That way the backlight will come on automatically anytime you want to actually look at your watch. If you find it dull indoors during the day, make sure Gesture is On, rather than just After Sunset.

In terms of the Casio, that is not a screen that is capable of running complex graphical elements like mapping. If you are only dealing with text, it's easier to come up with a low power screen readable in a range of conditions.