Peel the stencil from the chair. Give the entire chair a coat of water-based finish to protect the wood and the stenciled area.
If there's still some adhesive on the chair when the stencil is pulled off, gently scrape off the glue with a fingernail.
Stains can be mixed together so long as water-based is with water-based and oil with oil. Keep a record of the mixture, so it can be duplicated if desired.
To use multiple colors on one stencil, wait around 15 to 20 minutes. One of the advantages of water-based stains is that they dry very quickly, especially when it's warm and dry in the work area. If it's not, the process can be sped up with a hair-dryer on a low setting.
If finishes that come in aerosol cans tend to plug up the sprayer after the initial use, when spraying is done, turn the can upside down and press the tip until all the finish has been expelled and nothing is coming out but propellant. That step will blow the finish out, so it won't dry in the nozzle.