Is B3 blue the same as UU1 Blue?

That's not entirely correct.

69 and up, the first digit is the year of origin, the second and third digits are the color code. That's why it's important to distinguish the difference between GB5 and KB5.

68 and down, the PNT code on the tag likely will have two of the same letter and usually a "1". The first digit is NOT the first year the paint was available, like the 69 and up codes. For example- YY1 is the same color (but might be a different name) in 66 and 67, and there is no XY1 or ZY1.

IIRC a two tone car (say, a white hat special) would be WY2)

Note that numbers were also used- 881 (1968 and down) was very similar to EB5 (1969 and 1970).
The formula changed in 71 to GB5, and then in 74 to KB5.

The first letter is the same year code as engine block stampings.