Many reptiles, especially snakes, don't see red light. However, some see more than humans, such as bearded dragons who have yellow specific color receptors. What colors do Savannah monitors see?
Yes, monitor lizards can see red light. So can snakes, and other lizards, and birds, and turtles, and most mammals. However, not all of these animals can distinguish between what we would perceive as red and green light.
Now for a little background. Light is a spectrum, and neither humans nor any other animals can perceive the entire spectrum. Instead we have different receptors in our eyes that respond to different parts of that spectrum. Humans have three color receptors: what we call red, green, and blue. But since these receptors overlap quite a bit, you can't tell the whole spectrum just from what color you see. Something that looks yellow to you could be a mix of 550nm (green) and 600nm (red) light or a true 575nm (yellow) light.
What this means is that even if you didn't have a red receptor, you would still be able to see red light, but you would have a harder time distinguishing between what we would call red, green, and yellow.
Most mammals have two color receptors: blue and yellow.
Most reptiles, birds, and amphibians have four distinct color receptors, which vary a bit between species, but more or less correspond to what we would call violet, blue, teal, and yellow.
So, your monitor lizard can see red light (with their yellow receptor), but would have a hard time distinguishing it from yellow and green.
According to Professor Sam Sweet, "(savannah) Monitors are also specialized compared to other lizards in having acute long-distance (color) vision, sensitive hearing, and very well-developed senses of smell and tongue-based vomeronasal organ..."
It seems that savannah's have the entire color spectrum available to them.