If you use Philips hue bulbs thru the Philips Bridge connected to Hubitat, can you still use Hubitat apps Like Simple Lighting?
And if so, can you then also use ikea bulbs wich are not supported directly on Hubitat , like the RGB IKEA bulb or the IKEA tradfri motion sensor in Philips Hue Bridge, with Hubitat apps?
Thank you!
Yes to all questions except the Ikea motion sensor.
However, you CAN join Ikea bulbs directly to Hubitat as well and not both with another hub if you don't want to.
But I could't get the ikea RGB bulb to show colors, it's only white. So that's why I wanted to connect it to the Bridge.
Also, why can the Hue sensor be used but not ikea's since I get to connect it to the phillip hue bridge ?
Ahhh, the RGB bulbs... Correct. Not supported on Hubitat (sorry about that, I read too quickly). You can use them if joined to a Hue Hub with any of the Hubitat apps.
The Hue integration only supports bulbs and Hue groups for devices connected to the Hue Hub.
It can't. The Hue Bridge integration (I assume that's what you mean; the Hue sensor can pair fine directly to Hubitat) allows for control of bulbs or groups only, as stated above. Sensors are exposed through this API too, but it would be polling only (like a Hue bulb changed outside Hubitat is) and quite painful to use that way.
THANK you ALL for clarifying this for me. Now I wonder If I would be better of to just buy some other bulbs that work directly with HE instead of buying a HUE Bridge..
That's a mixed bag: most Zigbee bulbs are repeaters but repeat poorly for non-bulbs. Sengled is an exception because they don't repeat, and some have had good luck regardless of this fact with others (some do appear to be better, and some may be perfectly fine, but a few are known to be quite bad). Using a second Hubitat dedicated to bulbs is one way to solve this issue but not much more convenient than using a Hue Bridge. Most of my bulbs are Hue Bridge bulbs, and it works fine as long as you don't need instant status reports from your bulbs (or you change it from Hubitat directly if you do). Most motion sensors can be paired to Hubitat and used for Hue bulbs that way--regardless of where the bulbs themselves live--though Ikea's is likely an exception as it's probably designed for direct pairing to bulbs (something you could still do if both are paired to Hubitat and then you link them to the bulbs, but Hubitat won't be in the loop for its status).
You can connect the Hue Sensor directly to HE. This would require removing it from the Hue Bridge, and any Hue automations it connected to. However, you could likely recreate the same automations in HE, and include non-Hue devices in the automation.
Well, I guess I buy the Hue Bridge then.
How about this other question I wrote in a new post, Xiaomi AQara Smart Window Door Sensor are they working or not, on Hubitat?
I did't find them listed on compatible devices but some say they work..
So can I get a clear answer to know if I should buy it or not?
Thank you!
They are not officially supported, but many people have made them work. However, you have to be careful with how you build your Zigbee network: they do not work well with most repeaters and will "fall off" your network if they route through one.
You also need custom drivers, but that's the easy part. 
The rest Is summarized here: Xiaomi & Aqara Devices - Pairing & Keeping them connected. If you are totally new to Hubitat and Zigbee, I'm not sure I'd recommend them, but most people can get them to work.
I see, thank you! I will post an update if I get them
Download the Hubitat app
