@rautchetan this is certainly a difficult question to answer and difficult technique to master. When you are outside and you have to be against the light, then its of no use matching colors or values against the source. You simply can't as the brightness levels are different.
What you can do is to put a satisfactory mid tone value and then build your other values in accordance with the other elements in the painting. Sort of like a jigsaw puzzle. Squint often, if a portion of an object is too inward moving (darker than usual) then obviously it is wrong. If a section is too outward moving then it is obviously a lot brighter than usual. But this ability to judge values comes with a lot of practice and training of mind.
Mark's method simplifies this big problem of judging values by using lights with same brightness level onto the still life and painting. If you practice more from life, shadow box etc. It will help you greatly. Most of the time some of the gray tones will make you heavily confused, but you will learn to see.
Thanks everyone. I am not very skilled at this yet. What I am aiming at is to reach the high standards of 19th c artists. Their sketches were so truthful that sometimes they are treated as individual paintings. This is surprisingly missing in today's outdoor oil paintings and becomes somewhat forgettable.
Sketches and paintings of the past. These guys were trying to record a scene but while doing that these became so powerful.
But now it looks kinda like this; just a recording of an image with no interpretation whatsoever (with exceptions). Subject matters are always the same: