I have a code like below, and it works fine. It clears the screen, puts some color in color memory of first 12 characters on screen, and prints a text on the screen.
         jsr $e544
         ldx #$00
         lda #3
loopclr: sta $d800,x
         inx
         cpx #$0c
         bne loopclr
         ldx #$00
         lda #0
loop:    lda message,x
         sta $0400,x
         inx
         cpx #$0c
         bne loop
         rts
message: .byte "Hello "
         .byte "World!"
What I wonder is, if there's an easier way to change the text color in C64 Assembly, like POKE 646,color in BASIC?
Edit: I thought I need to be more clear, I can use
lda #color
sta 646
But it doesn't affect the text put on screen by assembly code in 1024+.
Is there an address that affects all characters put on screen?
Edit: I think I know the answer, no.
 
                        
Now there's a question I never thought I'd be asked! sta $0286 (646 decimal) sets the background color to be used when using the system print routine ($FFD2) which I recommend over direct access to the video ram since it takes into account the cursor position. So:
Well, there goes my credibility as a modern assembler guy! ;-)