OpenHAB items:

Switch Lamp_Switch "Lamp"

String Lamp_Red  "Lamp Red"

String Lamp_Green "Lamp Green"

String Lamp_Blue "Lamp Blue"

String Lamp_Brightness "Lamp Brightness"

OpenHAB Rules

rule "Turn Lamp off"
when 
    Item Lamp_Switch received command OFF
then
    sendHttpGetRequest("http://10.1.1.83/lamp/OFF")
end

rule "Turn Lamp on"
when 
    Item Lamp_Switch received command ON
then
    sendHttpGetRequest("http://10.1.1.83/lamp/ON")
end

rule "Change lamp brightness"
when
    Item Lamp_Brightness received command
then
    sendHttpGetRequest("http://10.1.1.83/lamp/brightness/" + receivedCommand)
    postUpdate(Lamp_Brightness, "")
end

rule "Change lamp red level"
when
    Item Lamp_Red received command
then
    sendHttpGetRequest("http://10.1.1.83/lamp/red/" + receivedCommand)
    postUpdate(Lamp_Red, "")
end

rule "Change lamp green level"
when
    Item Lamp_Green received command
then
    sendHttpGetRequest("http://10.1.1.83/lamp/green/" + receivedCommand)
    postUpdate(Lamp_Green, "")
end

rule "Change lamp blue level"
when
    Item Lamp_Blue received command
then
    sendHttpGetRequest("http://10.1.1.83/lamp/blue/" + receivedCommand)
    postUpdate(Lamp_Blue, "")
end

OpenHAB Sitemap

sitemap lamp label="Lamp"
{
  Frame label="Lamp controls" icon="lamp" { 
    Switch item=Lamp_Switch
    Switch item=Lamp_Brightness mappings=["DOWN"='-', "UP"='+']
    Switch item=Lamp_Red mappings=["DOWN"='-', "UP"='+']
    Switch item=Lamp_Green mappings=["DOWN"='-', "UP"='+']
    Switch item=Lamp_Blue mappings=["DOWN"='-', "UP"='+']
  }
}