I would paint the side door the same color as the siding or white to indicate it is a secondary entrance. I think most people can see that there is a main front door and, unless they are friends, will use the front main door.

Yes, paint it same color as siding. If this was my house, I think I’d be humorous about it and add a sign to the door saying “<<- front door :)”

That is a little odd. Usually set-ups like that have the door facing the rear yard, not the front yard. Might it be possible to do that instead? Close up the door on this side, then add it to the rear so it's easy for you to bring yard equipment out to the back, or allow your kids to come in to take off muddy/dirty stuff before they walk into the house. You could still add a window, if you like, or make the door to the back a full panel glass pane door for light.

Paint out the door so it doesn’t become a point of attention: use either the existing sandy colour or paint the whole wall to match the brick.

A much larger lantern over the door (wall-mounted or hanging) would draw attention to the front door.

This is very common where I live, actually. Many of my neighbors just decorate it as they would a front door. I can tell which is your actual front door as it's flanked by the columns. Although the light siding does draw attention to it. I like Patricia's idea... install a nice curvy sidewalk that leads to the front door. Perhaps off the sidewalk you can place large stepping stones to the mudroom door.

Could you install brick to match the rest of the house?

Paint that second front door and the whole surrounding area to match or blend into the brick. You could add a low porch rail with a gate to enclose the area further. And/or you can plant a low hedge along the porch. If people are still confused, a discreet red arrow painted on the door may work.

We have the same situation. I put a sign pointing to the front door. But delivery people still knock on the laundry room door or on our windows or even go in our backyard. One guy thought it was a duplex and went up the backyard deck steps to knock on our bonus room slider. IDK

Just be frank about it. Hang one of these on the door. Put two laundry tub planters right and left full of seasonal blooms or greenery.

Patton Decor Vertical Farmhouse Hand Painted Laundry Tin Enamel Sign · More Info

Vintage Metal Wash Tub Planter Sink · More Info

Rustic Galvanized Metal 2 Piece Round Wash Tub Planter Stand Set · More Info

This is very common with colonials. I couldn’t live without a mud entrance at least through the garage? The execution here is a bit off. I wouldn’t paint out your door to match the walls I don’t think. Your place needs some landscaping/hard scaping anyway. I agree that a more prominent path leading to the front door, with maybe a beautiful tree to somewhat obscure the 2nd entry and smaller stepping stones to it would work. Consult a landscape architect:)

This is super common in VA. How the front walk is designed makes a world of difference. Our "easy" trick is to make the front walk from the driveway wide, formal, and with a pattern that excludes the little entrance. Then we do as minimal a path as possible to that door. People figure it out pretty quickly.