When we choose 'Set Up Settings for an Image Brush', you'll notice that the 'Hue' property has a value of 'Tracing H' @100%, the 'Luminance' property has a value of 'Tracing L' @50%, the 'Saturation' property has a value of 'Tracing S' @100% and the Alpha property has a value of 'Tracing Alpha' @100%. This means that the sticker spray tool will trace the HSL and Alpha values that you choose. So if I click a blue on my colour picker it will spray blue. I can select colour in any of the usual ways with my eyedropper and the sticker spray will 'trace' these tint values and apply them to my spray.
Note: this only works accurately for stickers that were created with RGB 255,0,0 as we did with our Photoshop brush.