Dogs are dichromats which means they have two different colour receptors (we have three). They see the blue/indigo/violet range and the yellow-green range (so that’s the Y G BIV part of the old ‘roy g biv’ mnemonic device.) What they lack (and what we have) is the RO or red/orange receptors.
And grey. It seems grey is the base colour in any creature that uses receptors to see.
So, dogs see in a couple of shades each of blue, and yellow/green and grey. They see red and orange as ‘green’, anything of a greenish blue as gray and they struggle to see purple.
And so now we know!