"Roses are grey, violets are a different shade of grey, let's go chase cars.", this old joke by Bo Burnham perpetuates the myth that dogs are totally colour blind, they're not.
Animals including humans have light receptors in the back of the retina called cones. Humans have three distinct types of cones each of which is tuned to a different frequency of light. In our case, with our some 6 million cones in each eye, we can detect frequencies in the blue, green and red range, giving us the vibrant view we have of the world.
Dogs have far fewer cones and only two types, those for blue and green, and are lacking any detector for red. This results in a different perception of the colour spectrum than humans as seen in this chart. So that bright red ball we just thru for our ball crazy friend will be seen as the greyish green at the top of the chart, almost camouflaged on the yellow grass in the middle of the chart. Even at that, the much lower density of cones renders the colours much less vibrant so buddy's bright red ball may be almost invisible.
However, we don't need to feel to bad for them, since what they lack in colour perception they make up for in low light acuity. Another structure around the outside of the retina, called rods, are sensitive to low light levels but not colour. The ratio of rods to cones is about 2.5 times higher for dogs than humans, meaning they can detect objects in low light much better than humans. Another adaptation contributing to low light acuity is the larger pupil, allowing more light to reach the retina.
Still another adaptation, called the tapetum is a reflective layer behind the retina which reflects light back into the rods and cones. This is what causes animals eyes to shine in the dark when illuminated (this is different from red eye in flash photos of humans which happens because the iris cannot close quick enough to filter the light and we see the reflection of the blood rich retina). Humans do not have a tapetum.
So it turns out dogs do see colours, although not as vibrantly nor as many hues as we do but have built in night vision.
References: Are Dogs Colorblind? Elizabeth Palermo
Live Science June 27, 2014
Can Dogs See Colors, Stanley Coren PhD., DSc, FRSC
Psychology Today, Oct 20 2008
Do Dogs Actually Use Color Vision?Stanley Coren PhD., DSc, FRSC
Psychology Today, July 22, 2013
Doggone: Your Best Friend Is Red-Green Colorblind,Laura Geggel
Live Science Nov 8, 2017