There is an imbalance of perceived brightness with different colour LED being turned on which requires manual adjustment. For example when red colour is alone, the is relatively dimmer than yellow colour when red and green are on simultaneously. Because the later represents two light sources.
Thus I have fine tuned the later and reduced their power output to around 50% of their original duty cycle. And I did that for the rests too.
Now the brightness from different colours are quite even.
When all three colours are lit, the duty cycle can be reduced even further to 33%.
But there is another problem, the perceived brightness level is not linear in relation to the duty cycle. The relationship is rather a logarithmic one, which can be quite complicated,
Another more accurate way is to derive a method to directly measure the brightness using a lux meter and map out the relationship between power and brightness. One may find out that each colour may have its own graph, and combination of colour may have other graphs.
Even so, lux doesn’t directly reflect on the brightness perceived. A brighter blue LED may appear dimmer than a green one due to subjective perception of human eye.