I'm not exactly sure what you're asking so I'm unsure if this will help.
When I've made or altered panels, I usually provide myself a copy, which I can open for reference or undo an unwanted change.
When I start the actual painting in MSPaint, I usually "copy" a small square of the total black (the part that becomes open air in the sim), click on the "paint can" & turn the black portion to white so that I can better see the edge of cockpit panel, then paste my little black square in the upper left corner (make sure you've actually painted the very first, upper left pixel black or you may think you've gone blind in the sim). When I've ended my painting session, I click on the "eyedropper" & sample the little black square, select the "paint can" again and turn my white back to black.