Hi there, this is the original way Chris plays the song on the guitar. At least in 
the AOL sessions from 2003.

Since I dont know the name of the chords I will give you them here for the 
beginning, sorry for that...
Figured the chords out from the Session at AOL of the song
(search on youtube "Warning Sign - Coldplay Live Sessions @ AOL", gorgeous 240p video!)
garvmg from coldplaying.com helped me with the guitar tuning!

Tune your guitar the following:
EADGBD (high e string 2 steps down from E to D)
Capo 2nd fret!

Start: C 3.3.2.0.1.0    C# 3.3.0.0.1.0

Verse:                  Chorus:                    End:
C3.3.2.0.1.0 (x32010@1)          C3.3.2.0.1.0 (x32010@1)             G10.10.9.0.8.0 (320003@1)B8.8.7.0.6.0 (x2444x@1)          Em0.2.2.0.0.0 (022000@1)            B8.8.7.0.6.0 (x2444x@1)G10.10.9.0.8.0 (320003@1)        G#3.2.0.0.3.0 (xx1114@1)            F#1.3.3.2.1.0 (244322@1)D5.5.4.0.3.0 (xx0232@1)          D#0.0.0.2.3.0 (x43121@3)            Em0.2.2.0.0.0 (022000@1)
Here comes the Song:
-------------------------------------------------------------------------

C/C# (C most of the time, C# very shortly, just a finger lift from the third fret, 
see the video for the idea!)

BC GD      x4

B (x2444x@1)     C (x32010@1) A warning sign
G (320003@1)           D (xx0232@1)                   B (x2444x@1)  C (x32010@1) and I missed the good part and I realised
G (320003@1)         D (xx0232@1)                   B (x2444x@1)    C (x32010@1) that I started looking and the bubble burst
G (320003@1)         D (xx0232@1)                B (x2444x@1)  C (x32010@1)  G (320003@1)  D (xx0232@1) when I started looking for excuses

Come on in
and I ve got to tell you what a state I m in
yes I ve got to tell you in my loudest tones
that I started looking for a warning sign    
(continue on D here for a few seconds, then chorus!)

Chorus:
           C (x32010@1)       EmWhen (022000@1)the truth is
    G# (xx1114@1)      D#I (x43121@3)miss you
        C (x32010@1)          Emyeah (022000@1)the truth is
        G# (xx1114@1)     D#that (x43121@3)I miss you so

A warning sign
you came back to haunt me and I realised
that you were an island and I passed you by
when you were an island to discover

Come on In
and I ve got to tell you what a state I m in
yes I ve got to tell you in my loudest tones
that I started looking for a warning sign

Chorus:
When the truth is
I miss you
yeah the truth is
that I miss you so...
        C (x32010@1)      Emand (022000@1)I m tired
                  G# (xx1114@1)    D#I (x43121@3)should not have let you go...

G (320003@1)B (x2444x@1)F# (244322@1)Em (022000@1)   x2

G (320003@1)   B (x2444x@1)            F# (244322@1) EmSo (022000@1)I crawl back into your open arms      x4