With the final colour scale here

And try for 0 and 0.5 a lighter grey
[[0,[130,130,130]],
[0.5,[130,130,130]],

Thank you again for all this help :)

I come back on the upper end of the colour scale. I was not satisfied with the red colours and with 10 days accumulation in tropical area where it is necessary to have a semi logarithmic scale.
So I propose these steps:
…
300mm
400mm
700mm
1200mm

With the following code from 300mm:
…
[300,[251,40,251]],
[399,[251,40,251]],
[400,[240,0,50]],
[698,[240,0,50]],
[700,[210,11,40]],
[1150,[210,11,40]],
[1200,[143,11,40]]]

Today, as example, using this setting, on 10 days accumulation, in the East of Cairns ACCESS gives this map:

For the light grey, I used 170,170,170

I've just change the 0mm & 0.5mm RGB codes back to the default grey.

The new colour codes really help the different mm amounts pop.

Here is the code:

[[0,[255,255,255]],
[0.5,[255,255,255]],
[1,[247,185,95]],
[4.5,[247,185,95]],
[5,[253,175,42]],
[9.5,[253,175,42]],
[10,[254,252,55]],
[14,[254,252,55]],
[15,[180,253,51]],
[24,[180,253,51]],
[25,[86,253,48]],
[49,[86,253,48]],
[50,[35,228,155]],
[99,[35,228,155]],
[100,[30,167,252]],
[149,[30,167,252]],
[150,[65,72,251]],
[199,[65,72,251]],
[200,[176,37,251]],
[299,[176,37,251]],
[300,[251,40,251]],
[399,[251,40,251]],
[400,[253,80,156]],
[698,[253,80,156]],
[700,[244,10,75]],
[998,[244,10,75]],
[1000,[143,11,40]]]

You must modify the color settings on the website. That’s not possible from the phone app. But the same modification will be applied to all your devices.
Go to Settings
Scroll down and choose Customize color scale
Select overlay:
Rain accumulation
At bottom of page find the line Import/export
Click on [view code] to open a window pop up
Delete the text code in this window
Select and copy all lines of code here above and paste them in the window
Press [Import Gradient]
Press Save
You can come back to the default Windy settings at any time by pressing Load defaults

It gives this scale and map:

Hope you’ll use it.

If you want to get the code in order to use directly this colour scale, just let me know. That's easier instead of adding lines.
To make a comparison with this colour scale and your own scale, you may use the new one with a different browser without logging in.

Personally I would use the 400 - 500mm red for 500 -1000mm and vice versa.

Thanks. Here are my settings. I just estimated the colours. The upper reds were a failed attempt to make the legend more helpful.

@atmosj
There is no way to customise the steps of the colour scale.
But I don’t think that remembering 10 colours is so difficult. I have 13 for my own wind scale, and never use the legend.
As @Yves70 said, at the end, the weather picker is the best tool to know the exact rain accumulation in the areas where your colours show high values.
btw could you post a screenshot of a rain accumulation map with your customised colours. That should be interesting for Windy users.

Many thanks to you and @Yves70 for your responses and apologies for the late reply. I'm new to the forum and did not think there had been any replies. I do use the weather picker all the time, but find the fixed legend scale inelegant.

The legend sample I enclosed came from an accumulated rainfall forecast map provided by the Australian Bureau of Meteorology. They provide a 1-4, 5-8 and 1-8 day forecasts for our region using combined input from 5 models, including the ECMWF. They are very useful for identifying the potential for flooding from multiday rainfall systems like east coast lows and tropical cyclones.

To see the maps and more information about the forecasts, see:
http://www.bom.gov.au/fwo/IDYPME04/20210211_20210218/pme1to8.png

My 1-5 day equivalent Windy version looks like this: