Something ramshackle but undeniably power armour. And I don't think he'll have time to paint it any other color so it'll be just natural ceramite grey.
He isn't Guilliman, he doesn't care about spit and polish, at least among his own sons. Though he might "dress up" for the sake of the masses if it got the masses to fight harder.
Actually I was thinking he doesn't come back in armour at all, just leather and robes with lots of fur coverings to denote travelling. Normally you don't travel in full body armour.
I know Gandalf is inspired by Odin in the first place, but I also can't help thinking of a big buff looking Ian McKellen in robes of grey and furs flecked with white. Leaning on a great staff, but also with an ornate sword at his side (remember that Gandalf uses a long sword called Glamdring most of the time)
"The 6th Legion has a reputation." said Bear.
"All the Legions Astartes have reputations," replied Hawser.
"Not like ours," said Ogvai "We are known for our ferocity. We are thought to be feral and undisciplined. Even brother Legions consider us to be wild and bestial."
"And you're not ?" asked Hawser.
"If we need to be," said Ogvai. "but if that was our natural state, we'd all be dead by now."
He leaned down towards Hawser like a parent addressing a child.
"It takes a vast amount of self control to be this dangerous" he said.
Excerpt from "Prospero Burns" by Dan Abnett