Colouring cells according to date ranges

[Bananna1]

Hi

I have attached example.
In tab called 'actions list' there is a column called 'by when' which copies through into the 'gantt chart' tab
What I want to do is (in gantt chart tab) - when the 'by when' date is say, March - the columns Jan - March become coloured green; if the by when date is Oct, the columns Jan - Oct colored green.
Is this possible?
Thanks folks.

[Kaper]

Use conditional formatting based on formula.
for instance select E4:Pxxx and home->conditional formatting->new rule->use formula...

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and green fill

of course it can be developed further not to color if column C is not established
for instance with such formula:

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and so on and so forth (for instance in row 3 you have 1st days of month - to compare with end use EOMONTH ...

[Bananna1]

Thanks Kaper - that works brilliantly!

If it were to get more complicated...... is there a way to take into account the 'date open' (column D on first tab) so that if the colours on the gant chart only start in the month the date open lists (currently it is colouring everything from January - useful this month but obv as the year progresses, less so!

I've attached what I have done so far.

[Kaper]

So you shall add another part to your AND, for instance checking if a value in row 3 is bigger than

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(could be without this indeks/match if rows in this sheet follow arrangement of ActionList) - see corrected sheet.
To show given month if project starts during this particular month (like Jan 05, 2016), one shall use date function combined with Year and Month - see Gant chart (2) - here I assumed that arrangement of rows is identical, and not used index/match pair.

[Bananna1]

That's perfect - thank you for your help