I bought some Daiso acrylic ink after seeing it on social media.

They also sell metallic colors in packs of two for 100 yen, and they seem like they could be a good substitute for metallic Gundam markers.

The purple on the torso and shoulders is Copic, but the rest is Daiso pens. I think they're quite easy to use.

I read that the white doesn't have much opacity, but it covers quite well. (I used the white for panel lining in the gaps of the shoulder armor ducts.)

Daiso acrylic ink | ねこ's Gunpla works