I'm trying to use the aluminium complexation reaction for flavonoid content in wine but I find that the color of my solutions don't match with literature data and the sample doesn't absorb much at 510 nm (I have max at 415 nm).

First i add catechin standard solution then NaNO2 solution and after 5 minutes AlCl3. The solution should be yellow but mine is without color. After 6 minutes I add NaOH (1 M) and solution should turn immediately to red but my solution turns yellow no mater the concentration of the standard solution.

I'm using brand new chemicals, solutions are made just before analysis and I protect everything from light with aluminium foil.

What is the problem?