Make sure you're spelling the tag correctly (it's "Color" rather than "Colour")
Sorry, I did in the game; my browser's autocorrect is set to Canadian English.
Some commands to test are:
/summon area_effect_cloud ~ ~1 ~ {Particle:reddust,Radius:30f,RadiusPerTick:0f,RadiusOnUse:0f,Duration:1000,DurationOnUse:0f,Age:-3000,Color:65356}
And
/summon area_effect_cloud ~ ~1 ~ {Particle:mobSpellAmbient,Radius:30f,RadiusPerTick:0f,RadiusOnUse:0f,Duration:1000,DurationOnUse:0f,Age:-3000,Color:65356}
The Color
tag is for bright green, but the particles show up in default colours in both cases.
Also, once when I did it with reddust, there was a little poof of rainbow reddust particles in a circle when it spawned, but I haven't been able to reproduce that.
It would still be nice if there were a way to set a certain flag in map##.dat or something that would automatically fill it in using the heightmap style the same way that it is when generated by a cartographer villager
Sorry for wasting time by putting this in the wrong place.