i thought so, when you switch the 30 second layer off does the 5 min become alot more vibrant? if it does you need to use the Black brus as not to let any grey shadows come though, they will dull the overall image also with your bright stars cover them up so only the 5 min comes though that way your star will blend in nicely
I had a few problems with mine as i was blending 5 images together where it would dull the image but once i figured it out
happy days if you want have a look at my final process in my website.