Hi Thaimonk, this statue looks like the one available in the e-shop in this website, I bet you must have ordered it here. It looks really stunning when it is painted, Dorje shugden comes alive!
Painting statue in gold or in colors is a form of offering to the buddha. We should offer the best to the buddha always, since the buddha represents the body speech and mind we aspire to achieve.
So if you can afford real gold, then paint the statue in real gold. if not, you can use gold and any other color paints. Many people also like to offer jewels on the budda as an offering too.
So I guess there is no fix rule as long as it is offered sincerely from the practitioner.