Is there a photos brush to paint a plain colour background

It would be easier to use an external image editor. It would be easier to use a 3rd party image editor from within Photos. We can now edit from within Photos with a 3rd party editor like we did with iPhoto with the app, External Editors For Photos, that installs an extension which lets us chose any 3rd party editor of our choice to use within Photos for editing.

After running the app for the first time this is how you select the external editor(s). Select External Editors in the Edit window's Extensions menu:

and your editor of choice at the right:

You can choose any image editor that is in the Applications folder.

Here's what you need to know about editing with Photoshop or any 3rd party editor from within Photos:

Note: when I refer to the Photoshop file extension, .psd, it can be the file extension that your particular editor of choice uses.

1 - if you create a .psd file from a jpeg original you must save it to the desktop and import as a new file just like with iPhoto.

2 - if a jpeg is edited by PS resulting in a .psd file and it's saved without selecting the Desktop the default location will be here in a folder like this

AND the resulting folder and .psd file will be deleted automatically.

3 - if you flatten the file in PS and save it as a jpeg it will be saved correctly and recognized by Photos.

4 - if the original file is a .psd file and is edited by PS and just saved the resulting edited version will be converted to a jpeg file and recognized by Photos. Also if edited with PS a second time you'll lose the layers and just be editing a jpeg file.

5 - to keep the .psd file so you can do another edit first duplicate it and edit the duplicate file with PS. That will preserve the original .psd file and it's layers for additional edits even though the first edit will be lost to the second edit.

Which version of macOS are you using? In Sierra, you can also use the Markup extension to retouch a photo. Markup comes installed by the system, and you can access it from the Extensions item in the Edit panel in Photos without the need to call an external editor. See this Help page: https://help.apple.com/photos/mac/1.2/?lang=en#/phteb52f43ce

Once you have Markup enabled, you can draw lines and shapes on your photo or add text in any color you want.

The brush in Photos seems to pick up and paint faint copies of sections of the photo, not the area sampled. Is there a way to avoid this?

The retouch brush copies the area around the spot you sampled. If you want to copy only the exact spot, do not brush long brush strokes, but very short strokes. click again and again.

Or better use an external editor that allows to erase a region.

In Preview you can select a region and copy it onto a white background.

I am trying to mark replies as "helpful" but the buttons are dimmed and inactive.

Don't worry, this is normal. There is a limit to the number of "Helpful" you can award. For each discussion you can mark two replies as "Helpful" and one reply as "Solved". You've used up your supply of "Helpful" marks. Thank you for them 🙂

You have Preview - it comes on all Macs

In Preview you can select a region and copy it onto a white background.

LN

You are welcome

LN