Poster
MAIN CHARACTER
The guy on the poster is Charles Messier, wearing the Cross of the Legion of Honour.
FLYING CAR
Messier is riding a Messerschmitt bubble car. The bubble car came in a version with 'comet tail' trim, and there was a Messerschmitt ME163 'Komet' rocket-powered airplane in WW2. The car was modified to reference the Me-109 airplane, and the list of Messier Objects includes the M109.
COMET AND STARS
RAINBOW FOOTBALL BOOTS
There is a range of Messi football boots, named after the famous footballer Lionel Andrés "Leo" Messi Cuccittini. These come in rainbow colours as below:
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COLOURED SCALE STRIPS
Musical notes can be mapped to colours, as in this example. This is probably a better music/colour scale (and it is Newton's, which seems more in keeping...) This may be the conversion (top line), with each of the seven colours representing an interval in an octave:
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Might be worth trying reversing the scale (bottom row of the figure) if the first ordering doesn't make sense. The heights of the coloured sections are likely to represent the duration of each note; i.e. measuring them and getting the size ratios would enable translation into minims, crotchets, quavers, breves, etc.
Guilielmus_Messaus is a Mess... composer, noted for Christmassy tunes. There's also Olivier Messiaen and Handel's Messiah. Tempting to think of a play on words messier comet - messiah cometh...?
NOTATION ON COLOURED SCALE STRIPS
V.2
LL. 3-4
This could be a biblical reference.
RR (crossed through) SS
M (crossed through) PL
+ sign
EA (crossed through)
TY's (crossed through) RIED
ANGELS AND OTHER CHURCH/STAINED GLASS IMAGERY
There are three angels in the rounded windows left and right (same image, flipped horizontally). The central angel looks like it is carrying a stone block, or possibly a musical instrument - like these angels here. 'Angel' derives from Late Latin 'angelus' (borrowed from Greek aggelos/ángelos), see Wikipedia; it means MESSenger.
CHURCH BUILDING
May be based on a real church/place of worship?
COINS SPILLING DOWN
May just denote treasure, since it is depicted as spilling from a pot at the end of the rainbow.
TITLE WORDING AND COLOURING
GET READY FOR HOURS OF MADDENING FUN, AS YOU SOLVE PUZZLES,
DECIPHER INSTRUCTIONS AND FIND HIDDEN TREASURE
STARTS ON 15 DECEMBER SEE PABLOSATH.COM
May not be significant.