Aside from colors reflected by plants, these other features could be signs of life:
Oxygen (O2) plus water (H2O). Even on a lifeless world, light from the parent star naturally produces a small amount of oxygen in a planet’s atmosphere by splitting water vapor. But the gas is quickly rained out, as well as consumed through oxidation of rocks and volcanic gases. Therefore, if a planet with liquid water has abundant oxygen, some additional source must be producing the gas. Oxygenic photosynthesis is the leading candidate.
Ozone (O3). In Earth’s stratosphere, radiation splits apart oxygen, which then recombines to form ozone. Together with liquid water, ozone is a strong biosignature. Whereas oxygen can be detected at visible wavelengths, ozone can be detected at infrared wavelengths, which is easier for some telescopes.
Methane (CH4) plus oxygen or seasonal cycles. Oxygen and methane are an awkward chemical combination that is hard to achieve without photosynthesis. A seasonal cycle of rising and falling methane concentrations is also a good sign of life. On a dead planet, methane levels are fairly constant, declining slightly over the long run as starlight splits the molecules.
Methyl chloride (CH3Cl). On Earth this gas results from the burning of vegetation (mainly forest fires) and from the action of sunlight on plankton and seawater chlorine. Oxidation destroys it. But an M star’s relatively weak radiation might allow the gas to build up to detectable amounts.
Nitrous oxide (N2O). When plant matter decays, it releases nitrogen in the form of nitrous oxide. Abiotic sources of this gas, such as lightning, are negligible.