When you use two describing words, how big comes before what color.
Size before color
a blue small phone → a small blue phone
a green long umbrella → a long green umbrella
a white big dog → a big white dog
Not "a blue small phone" — say "a small blue phone". Say the size (small) first, then the color (blue).
Try it: Put the words in order. Answer: a big red bag — Size before color: a big red bag. If you put red first, you named the color before the size.
Which word is size, which is color
Size words say how big: big, small, long. Color words say what color: red, yellow, blue, green, brown, black, white. Size first, then color.
small (size) + white (color) → a small white cat
big (size) + green (color) → a big green book
long (size) + black (color) → a long black umbrella
Try it: Put the words in order. Answer: a small blue bag — "Small" is the size and "blue" is the color, so size goes first: a small blue bag (not "a blue small bag").
Try it: Size first, then color. Which one sounds right? (a big black dog / a black big dog) Answer: a big black dog — Size (big) comes before color (black): a big black dog. "A black big dog" puts them the wrong way around.