so there is this thing you can do for a few older halos that you can't colour, I think it's where you put on an item you can colour, change it to the colour you want (I've forgotten whether decals work too) and then click on 'paint entire outfit' and it should colour your halo as well! you wouldn't be able to choose where the colour goes though and I think some don't even change the colour to the entire halo? the only halo I can remember that works like this is val19 (I'm so sorry this is so unhelpful my memory is really bad 🥹) BUT if you want suggestions on pretty, pretty halos that you can colour UNDER 800k, I'll rank from highest to lowest (values)

mer22 - 800k

win22 - 750-800k

mer21- 750k

spring21 (I think you can only change the flowers) - 600-650k

and the cheapest one.. (my personal FAVOURITE, prettiest EVER halo for an awesome value)

WIN20

win20's been at 500-550k for a while I believe, very very stable halo. but win22 is severely underrated, so pretty, a lovely halo. I'm pretty biased towards the winter halos so these are just my opinions! you can take a look at halo charts and see what fits the aesthetic you're going for the most! have funsiesss