zenith crts

[sampson159]

i have 3 zenith crts. 25vs with green label,blue and purple.they came out of the same chromacolor 2 sets. does anyone know why the different color labels? the numbers are all the same too. we jigged them up on a test chassis and noticed the purple label crt seemed to have the best quality picture. also the screen on it was darker the the other two. all tubes checked about the same on the beltron. green label looks to have a safety glass. any info?