Change color of the text written in PPU memory (NES-Super Mario Bros)

Instead of changing the palette itself you might find it easier to change the attributes instead.

Try this:

memory.writebyte(0x2001, 0x00) -- Turn off rendering
memory.readbyte(0x2002) -- PPUSTATUS (reset address latch)

memory.writebyte(0x2006, 0x23) -- Selecting high byte attribute table
memory.writebyte(0x2006, 0xC1) -- Selecting low byte attribute table 
; you might need to fiddle with this number, should be somewhere between 0xC0 and 0xC8
memory.writebyte(0x2007, 0x00) -- Use Palette 0

memory.writebyte(0x2006, 0x20) -- PPUADDR high byte
memory.writebyte(0x2006, 0x2A) -- PPUADDR low byte
memory.writebyte(0x2007, 0x14) -- PPUDATA ('K')
memory.writebyte(0x2001, 0x1e) -- Turn on rendering

Equivalent assembly code if you prefer:

LDA #$00
STA $2001
LDA $2002

LDA #$23
STA $2006
LDA #$C1
STA $2006

LDA #$00
STA $2007

LDA #$20
STA $2006
LDA #$2A
STA $2006

LDA #$14
STA $2007

LDA #$1E
STA $2001

Look at it in the Mesen emulator, under Debugger / PPU Viewer / Palette Viewer (there you can change the palette to see the effect).

For the text color, you want palette address $3F09. This is also the cloud's color. $3F11 is in the sprite palette rather than the background, and it seems to be Mario's outfit color.

Also, note that most games store the palette in RAM, and write to the PPU every vblank. Makes it easier to handle palette cycling, transitions, stuff like that.