When I first learnt to paint I was given a great tip to look through red cellophane to see tones and block out distracting colour. I used a tatty piece of red cellophane for a long time before having these acrylic viewers made up to share with my students at workshops.
They are great for using outdoors when trying to judge tricky values, like sky vs water reflections etc. I also use it to view my painting outdoors to see if the contrast in dark, mid and light values is showing up.
It has a hole in one corner that can be used for string or a lanyard.
Price includes free shipping in Australia.