Solved

Design point colours mean

What is the meaning of grey colour for design point and what is the ideal number for design point in rookie, pro, n elite ?

Not sure what the significance of the grey color is, never really thought it about but I'd reckon it's just a pointer that you've gotten to an 'okayish' place in the development of that attribute.

Ideal number for design points:
Rookie: 50 soft capped (suppliers can increase the number)
Pro: 100 soft capped (suppliers can increase the number)
Elite: 200 hard capped (no attribute can exceed 200)

I've always taken it as Red, not a attribute you should rely on. Grey, it's ok, but not good.Could be relied upon if had to.  Green, strength of the car.
That said, working out car design, this isn't really relevant as a guide.

Ok guys, thanks for that! Its green for acceleration tomorrow!

In short

Red = bad
Grey = meh
Green = good

Ignore the colours, they don't matter and nor does a low number in a car attribute. Only matters relative to the design paths other cars take in your league and the upcoming tracks.

So really don't focus on the colours, only develop in the stats that matter relative to the situation you find yourself. A red stat can both be not worth developing and at the same time be good for your league and a green stat can still be bad for your league.