Mix paint using red, yellow and blue give Hex code
Re-think this. It's not going to work.
You're starting by eye anyway if you're picking from a colour on-screen. Unless your display is fully calibrated using a hardware colorimeter* you already don't know what colour it should be. Most consumer displays are set up intentionally punchy &amp; over-contrast. This makes them look better in the shop. Setting your computer to the 'correct' profile provided by the manufacturer will not get you really much closer. You must use a colorimeter.
Ink/paint mixing doesn't use Blue, Red, Yellow, Black it uses Cyan, Magenta, Yellow &amp; Black. The primary colours everyone was taught at school don't actually work in practise.
The conversion from RGB to CMYK requires not only your display to be accurately profiled, but also your output 'printer', because you are always expected to be 'printing' on a white substrate. Profiling, even on a CMYK colour-match system such as Pantone, depends also on the paper quality, reflective quality &amp; absorbency.
Finally, you cannot mix under anything except calibrated light. If you don't have calibrated light, the best you can achieve is to go outdoors on a cloudy day. No domestic interior lighting is anywhere close to being accurate enough to attempt this task.
You also cannot know what colour paint will dry to based on a wet mix.
By the time you lump all those inaccuracies together you would be far better off going to your local paint store &amp; selecting a paint chip. Get them to mix it. Their systems are fully calibrated to work from a wet mix to when the paint is dry.
Buy more than you need, because even on a fully-calibrated system, there will be batch discrepancies [this is why when you buy pre-mixed paint, you make sure all the labels say they're from the same batch].
*$£€ 250 for a decent one, already 10 times the cost of your paint.
You can convert RGB colour to RAL colour using such as https://rgb.to - however you are still not going to know if your RGB was correct in the first place. The RAL Colour Standard is used for information defining standard colours for paint and coatings and is the most popular Central European Color Standard used today.