Can anyone optimise this code chunk?

No reason to set r,g,b to zero. They are initialized zero.

No reason to call ColorRed(), ColorGreen(), ColorBlue() in your decimal (DEC) statement because the values are already stored in r, g, b.

So your function could look like:

Function print_debug_info() 
r=ColorRed() 
g=ColorGreen() 
b=ColorBlue() 
Cls 
Print "PlayField Width " + GraphicsWidth() 
Print "PlayField Height " + GraphicsHeight() 
Print "PlayField Depth " + GraphicsDepth() 
Print "PlayField Memory " + GraphicsBuffer() 
Print "PlayField Drawing Color (DEC) " + r + " " + g + " " + b 
Print "PlayField Drawing Color (HEX) " + Hex$(r) + " " + Hex$(g) + " " + Hex$(b) 
End Function