Lemon blue would be pretty, but I'm not sure you could make it from buff. I think lemon blue has a different diluter. The yellow is a bright lemon rather than a rich gold. You'd need to have the genes that restrict the lemon colouring to the proper locations. I'm not sure, but I think for lemon blue you would need E^R base.
Blue + buff will dilute the black pigment in the offspring who receive the blue gene. Buff offspring who would have black in the tail or wings will have a lighter, less noticeable blue. However, not all the offspring will be buff; you will get some blues and some who are an unpredictable mix. Some VERY nice buffs and blues have been bred using this cross, but there are also a lot who didn't make the grade.