2: It adds depth and 3-Dimensional to the painting and I could refer to the artists picture plane, white space is known as positive space when there is primary objects occupying it. A positive and negative space can affect how good the artwork will come out.
3: Colour can show if it is bright or if it's dull, warm or cool and it depends on what end of the colour spectrum they are. purity or strength of a colour also matters, if you put to much or to little it will affect how you wanted the outcome to be.