Error 6. Condition in IF, WHILE, FOR statements has to be Numeric or Boolean type. You can not use array here, please use [] (array subscript operator) to access array elements

The if keyword executes statement1 if expression is true (nonzero); if else is present and expression is false (zero), it executes statement2. After executing statement1 or statement2, control passes to the next statement. Expression must be boolean ( True/False) type (so it CANNOT be ARRAY because there would be no way do decide whether to execute statement1 or not, if for example array was: [True,True,False,.....,False,True] )

if( expression )
  statement1;
else
  statement2;

EXAMPLE

if( Close > Open ) // WRONG
    Color = colorGreen; //statement 1
else
    Color = colorRed; //statement 2

Plot(Close,"Colored Price",Color,styleCandle);

The above example is wrong, as both Open and Close are arrays and such expression as Close > Open is also an ARRAY. The solution depends on the statement. It’s either possible to implement it on bar-by-bar basis, with use of FOR loop:

for( i = 0; i < BarCount; i++ )
{
   if( Close[ i ] > Open[ i ] ) // CORRECT
       Color[ i ] = colorGreen;
   else
       Color[ i ] = colorRed;
}

Plot( Close, "Colored Price", Color, styleCandle );

It is also possible in this case to use IIf( ) function:

Color = IIf( Close > Open, colorGreen, colorRed ); // ALSO CORRECT - working directly on arrays
Plot( Close, "Colored Price", Color, styleCandle );