Why the extracts of some natural dye, which are obviously colored, would not absorb in the visible ranger (those have large gap between the LUMO and the HOMO, indicating absorption in the UV region and not in the visible range)
I agree fully with the answers of Peter and Roman. Please give us infos about the conditions of extraction (water, ethanol.... at room temperature or boiling point of H2O or EtOH) . and reformulate your question.
The conjugation, responsible for VIS absorption, can be destroyed during the extraction at high temperature.
It seems that you are not familiar with internet searchs. Please start GOOGLE SCHOLAR and not PUB MED or GOOGLE.
give : "extraction of natural dyes" . You obtain 700.000 hits in 0,07S !!!!!!!!!!!!!!!!
Here the method found in Google Scholar related to Resaerch Gate.