Producing Colored Noise from a given PSD Data
Estimating the necessary filter order
There are methods for estimating the needed filter order for low/band/high-pass filters, but the fact that you are talking about &quot;colored noise&quot; suggests that you are not going for relatively simple frequency profiles.  Thus, the answer is &quot;no&quot;, I am not aware of an order estimator that will help you.  The best generic advice I can give you is to go as high as you can stand performance-wise and hopefully that will be enough.

Filtered/Colored output has lower amplitude than input
It sounds like you know the frequency profile you want, and are creating a filter that produces that profile.  If that is the case, fixing this should be easy.  Do the following:

Calculate the average of the amplitude of the frequency profile you are creating.
Divide all of your filter coefficients by that average.



That should make the overall gain of the filter &quot;1&quot; with regards to white noise input.