> Why don't "tourism people" just pay people constructing buildings to use the colour "tourism people" want?

That implies they could ignore that rule at any time. Reimbursement programs are an increase in paperwork, which many would simply ignore for convenience. This would give the town the "Tragedy of the Commons" problem, erasing it's historic sense (and primary revenue source), and it would become another run-down town like many others in the region.

Is that fair to those who invested heavily in keeping their businesses and homes in that area? Their answer is a resounding "No"

If somebody balks, just like this, there are other more modern and relaxed cities within a few miles that can accomodate their building ideas. These cities even have more access to freeways and trains, so economically it makes sense to put their businesses inside those cities.

Instead, the primary draw towards this town IS it's historical authenticity, and thus the people living there keep it maintained through it's building codes. There is no other reason why a business or homeowner would live in that area, so it makes sense to keep with it's character. If that's too onerous, then perhaps your motivations for building should be reexamined.