what i do to dodge that problem is to switch out using unicode blocks to image blocks of your desired colour! i've done it in my current sig instead of having two rows of unicode blocks! (under the links section)
the best way to do this is to:
1. search up a colour of your choice
2. take the image of that colour that comes up
3. crop to size
you could also take a screenshot of the current unicode box you have and
crop that to size! but i find that they get a bit blurry that way.
and there you have it! you could also probably make it mixed coloured (like glitter's unicode box) by going into an app like picsart and editing a lot of squares of different colours together! and then once again, resizing it again.
but yeah! that's the method i've been using for quite a while! makes sigs a bit more compatable across platforms.