Whole sidewalks is probably an exaggeration, your professor was probably referring to the painted kerbstones/curbstones that mark many loyalist and nationalist neighborhoods. These neighborhoods also paint lampposts, mail boxes, traffic lights, bollards, and trash receptacles. They're not simply green or orange, rather they are patterned after the respective flags. Loyalists would have red, white and blue painted curbstones based on the Union Jack, while nationalists would paint theirs green, white and orange for the Irish tricolor. Municipal authorities are not the ones who do the painting, but rather the people within the neighborhoods. It isn't always about marking "safe" territory, so much as it is about marking territory, period. It is a warning to the other side in many cases.

It is also worth noting that not all Northern Irish neighborhoods have these sectarian markings. Most tend to be located along sectarian flashpoints in working-class neighborhoods--so you would see them in the most divided regions of Northern Ireland. For example, "Free Derry" has a fair number of curbsides that are painted green, white, and orange. Another example, you would see a lot of red, white, and blue curbsides in East Belfast. They function as informal borders.

This all ties in quite nicely with other visual displays in Northern Ireland which can take the forms mentioned above. But can also take the form of murals, parading, pageantry, memorial gardens to fallen paramilitaries, and monuments. The curbstones serve as the most obvious and visible markers; they are located along the entrance to the neighborhood unlike murals which would be located a bit further inside the neighborhood.

This album of photos from Derry/Londonderry in 2009 provide some visual examples of this phenomenon. The bottom photo showcases how a loyalist neighborhood marks itself; the curbstones resemble borders and there are more prominent loyalist symbols like the Ulster flag and the Union Jack. Additionally, you can almost see the banners between the buildings, also in red, white, and blue. There is also a UFF (Ulster Freedom Fighters) logo sprayed on a building as well as two murals, the third photo is a close-up of one of the murals and features a painted curb and lamppost. Photos #1 and #2 are from the Bogside (or Free Derry) a nationalist stronghold in Derry/Londonderry. They utilize some of the same visual strategies as their loyalist rivals' neighborhoods. The lampposts at the Bogside Inn feature the Irish tricolor and RIRA (Real IRA) is tagged on the building much like UFF. The mural in photo #1 commemorates the martyrs from the Battle of the Bogside, a riot between the nationalists/marchers and the Royal Ulster Constabulary and local unionists in 1969. This mural also has a tricolor lamppost much like the BUA one.

All of this underscores that despite the deep divisions of sectarianism, these neighborhoods show a surprising degree of overlap in how they present themselves and mark their own turf.

Sources

Abshire, Jean E. "Northern Ireland's Politics in Paint." Peace Review 15, no. 2 (2003): 149-161.

Jarman, Neil. Material Conflicts: Parades and Visual Displays in Northern Ireland. Oxford: Berg, 1997.