ON ERROR GOTO ETrap     'set an error trap
SCREEN 12       'set the screen mode
PALETTE 1, 0    'assign black to color attribute #1 to use as the default

w% = 100: x% = 50         'set the viewport boundry coordinate var's
y% = 540: z% = 300
VIEW SCREEN (w%, x%)-(y%, z%), 0, 15    'define a viewport w/ border
LINE (w%, x%)-(y%, z%), 1, BF           'draw a box, fill w/ color 1

COLOR 15        'put options on the screen
LOCATE 3, 14: PRINT "PALETTE VALUE:"
LOCATE 21, 14: PRINT "R = More red                         Red intensity:"
LOCATE 22, 14: PRINT "r = Less red"
LOCATE 24, 14: PRINT "G = More green                     Green intensity:"
LOCATE 25, 14: PRINT "g = Less green"
LOCATE 27, 14: PRINT "B = More blue                       Blue intensity:"
LOCATE 28, 14: PRINT "b = Less blue"
LOCATE 30, 35: PRINT "Esc = Quit";

DO  'loop here and update the palette and data w/ each key hit

    a& = (65536 * blue%) + (256 * green%) + red%   'calc the new palette

    PALETTE 1, a&   'display the new palette

    LOCATE 3, 28: PRINT a&; "     "   'update the screen data
    LOCATE 21, 65: PRINT red%
    LOCATE 24, 65: PRINT green%
    LOCATE 27, 65: PRINT blue%

    DO: k$ = INKEY$      'wait for a user key
    LOOP WHILE k$ = ""

    SELECT CASE k$       'process the key
        CASE "R"
            IF red% < 63 THEN        'increment red intensity
                red% = red% + 1
            ELSE
                SOUND 200, .1
            END IF

        CASE "r"
            IF red% > 0 THEN         'decrement red intensity
                red% = red% - 1
            ELSE
                SOUND 200, .1
            END IF

        CASE "G"
            IF green% < 63 THEN      'increment green intensity
                green% = green% + 1
            ELSE
                SOUND 200, .1
            END IF

        CASE "g"
            IF green% > 0 THEN       'decrement green intensity
                green% = green% - 1
            ELSE
                SOUND 200, .1
            END IF

        CASE "B"
            IF blue% < 63 THEN       'increment blue intensity
                blue% = blue% + 1
            ELSE
                SOUND 200, .1
            END IF

        CASE "b"
            IF blue% > 0 THEN        'decrement blue intensity
                blue% = blue% - 1
            ELSE
                SOUND 200, .1
            END IF
       
        CASE CHR$(27)

        CASE ELSE
            SOUND 200, .1

    END SELECT
LOOP UNTIL k$ = CHR$(27)    'exit if escape is hit

VIEW        'close the viewport
CLS
PALETTE     'reset the palette to default
SCREEN 0

'print the final palette data
PRINT "FINAL PALETTE VALUE   :"; a&
PRINT "RED INTENSITY         :"; red%
PRINT "GREEN INTENSITY       :"; green%
PRINT "BLUE INTENSITY        :"; blue%

Done:
END

ETrap:
    CLS     'display the error code and exit program
    PRINT "BASIC RUNTIME ERROR #"; ERR
RESUME Done