Very good! Add color to first PSET

SCREEN 12
'c=colour
'x=pixelsFromLeft
'y=pixelsFromTop
'z=pixelsTall
c = 12
x = 20
y = 20
z = 20

'outline:
PSET (x, y), c 'add color
FOR i = 1 TO z - 1 'subtract one from depth for line at bottom
PSET (x - i, y + i), c
PSET (x + i, y + i), c
NEXT i
LINE (x - i, y + i)-(x + i, y + i), c 'add line for bottom

K$ = INPUT$(1)

'full
PSET (x, y), c 'add color
FOR i = 1 TO z
LINE (x - i, y + i)-(x + i, y + i), c
NEXT i

I thought you wanted ASCII text, but you figured it out just fine.
When the FOR loop exits, i is increased by one after the loop count.

Ted