There isn't much color variation in wood (at least not those that would have been available in Finland at the time), although there are some slight ones, so it would be a matter of hue rather than color, and would increase item count (not to mention having to deal with displaying mixing of wood in items such as walls, fences, and doors). Brightly colored trees are nice in games not rooted in realism.
Mining is not thought to have been practiced in the area at the time. The somewhat more advanced Vikings used bog iron, not mining from mines, so there's no reason to assume the iron age Finns used mines.