I don't think they are colour blind, show mini a green leaf then show her a red leaf and she will go for the red, as she knows it's a flower that she loves,
I think there has been a lot of observation of this subject with tortoise keepers. It's generally accepted that tortoises can see colors or at the very least, different shades. I've seen mine clambor right up to bright objects but once they get there they will always stop and then sniff the object. If it smells like food they'll usually take a bite, if they don't smell something familiar (or enticing) they'll usually give it a couple of more sniffs just to make sure and then turn around and walk away.
I think they rely on sight to first find an object at a distance and then once they get to it use their nose to decide if it's edible or not.
My Sulcata definitely see's colors, if he is eating and I put a Hibicus close to him, he will stop eating and go after the red flower and then go back to eating. he also has a thing for orange shoes...
Tortoises have 4 kinds of cones in their eyes, and cones are structures that react to color.
Humans have 3 kinds of cones- one each that reacts to red, blue, and green. This is why TV sets and monitors can make a full range of color with just those three color tubes, or why you can overlap red, green, and blue gel filters over a light to get a white light to human eyes.
Tortoises have an extra cone compared to humans. The best we can tell, it responds to UV light- probably a lot like an insect's eye does. they probably see all kinds of cool colors and patterns in plants, etc. that we are blind to, and probably also use this to find good basking sites and probably enhanced vision in low-light situations.
You know how when you are in a place with kind of low lighting and some UV/black lights on, and how odd things glow in that light? That is probably a hint of what tortoises can see.