Show us a small code snippet illustrating your problem, please.

is my problem similar? (I've just started learning processing)
Running a code from I. Greenberg's book, p.195 'Drawing lines with pixels example' using line:
PImage img = createImage(width, height, RGB);
it draws black lines in on black background

The only possible format parameters for createImage should be only RGB, ARGB, or ALPHA. Still, I replaced 'RGB' with '255':
PImage img = createImage(width, height, RGB); (which should not make sense?)
Processing draws image as shown in the book: black lines on white background.

complete code is below. Thanks.

size(500,300);
background(255);

//used by diagonal lines
float slope = float(height)/float(width);
//this is the line where I replaced 'RGB' with '255')
PImage img = createImage(width, height, RGB);
color c = color(0,0,0);

//horizontal line
for (int i=0; i<width; i++){
 img.set(i, height/2, c);
}

//vertical line
for (int i=0; i<height; i++){
 img.set(width/2, i, c);
}

//diagonal line (TL-BR)
for (float i=0; i<width; i++){
 img.set(int(i), int(i*slope), c);
}

//diagonal line (BL-TR)
for (float i=0; i<width; i++){
 img.set(int(i), int(height-i*slope), c);
}
image(img, 0, 0);

Mikhail, first, the format parameter of createImage isn't strongly checked by Processing: if you pass "garbage" there (as long as it is some int), it will try and use some default behavior (perhaps RGB, but get() will always return 0, etc.). Just stick to documented values...

Next, you should use background() to... change the color of the background. But first, you must switch from PImage (most used to store images loaded with loadImage or very simple pixel per pixel stuff) to PGraphics. Or do a loop to change the color of all pixels before drawing your stuff.
Code:

PGraphics img = createGraphics(width, height, P2D);
img.background(255); 

BTW, your problem is not similar, since you don't use loadImage()...