Return to Answer

void loop()
{
 decode_results results;
 if (!irrecv.decode(&results))
 return;
 // If the LEDs are OFF, the only valid action is to turn them
 // ON.
 if (!leds_are_on) {
 if (results.value == CODE_ON_OFF) {
 digitalWrite(RED_PIN, HIGH);
 digitalWrite(GREEN_PIN, HIGH);
 digitalWrite(BLUE_PIN, HIGH);
 leds_are_on = true;
 }
 else {
 irrecv.resume();
 return;
 }
 }
 // The LEDs are ON: all codes are valid.
 switch (results.value) {
 case CODE_ON_OFF: // turn OFF
 digitalWrite(RED_PIN, LOW);
 digitalWrite(GREEN_PIN, LOW);
 digitalWrite(BLUE_PIN, LOW);
 leds_are_on = false;
 break;
 case CODE_LESS_RED:
 r_bright = constrain(r_bright - 255/steps, 0, 255);
 analogWrite(RED_PIN, r_bright);
 break;
 case CODE_MORE_RED:
 r_bright = constrain(r_bright + 255/steps, 0, 255);
 analogWrite(RED_PIN, r_bright);
 break;
 // etc...
 }
 irrecv.resume();
}

void loop()
{
 decode_results results;
 if (!irrecv.decode(&results))
 return;
 // If the LEDs are OFF, the only valid action is to turn them
 // ON.
 if (!leds_are_on) {
 if (results.value == CODE_ON_OFF) {
 digitalWrite(RED_PIN, HIGH);
 digitalWrite(GREEN_PIN, HIGH);
 digitalWrite(BLUE_PIN, HIGH);
 leds_are_on = true;
 }
 irrecv.resume();
 return;
 }
 // The LEDs are ON: all codes are valid.
 switch (results.value) {
 case CODE_ON_OFF: // turn OFF
 digitalWrite(RED_PIN, LOW);
 digitalWrite(GREEN_PIN, LOW);
 digitalWrite(BLUE_PIN, LOW);
 leds_are_on = false;
 break;
 case CODE_LESS_RED:
 r_bright = constrain(r_bright - 255/steps, 0, 255);
 analogWrite(RED_PIN, r_bright);
 break;
 case CODE_MORE_RED:
 r_bright = constrain(r_bright + 255/steps, 0, 255);
 analogWrite(RED_PIN, r_bright);
 break;
 // etc...
 }
 irrecv.resume();
}

Edit: After reading your code, I have a few suggestions for making it clearer are more efficient:

1. Declare all the constants with the const keyword, at the beginning of your program. A common convention is to write them in all caps with underscores, to distinguish them from variables.
2. Avoid "magic numbers" within your code (e.g. the IR codes), and use properly named constants instead.
3. before is hard to understand, give it a name that makes sense. I suggest leds_are_on. And make it a bool, because it can only be either true or false.
4. One way to avoid deeply nested loops is to use return statements to get out of loop() as soon as you know you have nothing to do.

With these suggestions, I would start the program like this:

#include <IRremote.h>
// Pinout.
const int RECV_PIN = 12; // data out of IR receiver
const int RED_PIN = 11;
const int GREEN_PIN = 10;
const int BLUE_PIN = 9;
// Number of steps between black and full brightness.
const int STEPS = 5;
// IR codes.
const uint32_t CODE_ON_OFF = 0xb54a3ac5;
const uint32_t CODE_LESS_RED = 0xb54a8a75;
const uint32_t CODE_MORE_RED = 0xb54a7a85;
const uint32_t CODE_LESS_GREEN = 0xb54ab24d;
const uint32_t CODE_MORE_GREEN = 0xb54a32cd;
const uint32_t CODE_LESS_BLUE = 0xb54a0af5;
const uint32_t CODE_MORE_BLUE = 0xb54aca35;
const uint32_t CODE_YELLOW = 0xb54aa956;
const uint32_t CODE_PURPLE = 0xb54a2ad5;
const uint32_t CODE_AQUA = 0xb54aea15;
bool leds_are_on;
int r_bright;
int g_bright;
int b_bright;
IRrecv irrecv(RECV_PIN);

and the loop() would be like this:

void loop()
{
 decode_results results;
 if (!irrecv.decode(&results))
 return;
 // If the LEDs are OFF, the only valid action is to turn them
 // ON.
 if (!leds_are_on) {
 if (results.value == CODE_ON_OFF) {
 digitalWrite(RED_PIN, HIGH);
 digitalWrite(GREEN_PIN, HIGH);
 digitalWrite(BLUE_PIN, HIGH);
 leds_are_on = true;
 }
 else {
 irrecv.resume();
 return;
 }
 }
 // The LEDs are ON: all codes are valid.
 switch (results.value) {
 case CODE_ON_OFF: // turn OFF
 digitalWrite(RED_PIN, LOW);
 digitalWrite(GREEN_PIN, LOW);
 digitalWrite(BLUE_PIN, LOW);
 leds_are_on = false;
 break;
 case CODE_LESS_RED:
 r_bright = constrain(r_bright - 255/steps, 0, 255);
 analogWrite(RED_PIN, r_bright);
 break;
 case CODE_MORE_RED:
 r_bright = constrain(r_bright + 255/steps, 0, 255);
 analogWrite(RED_PIN, r_bright);
 break;
 // etc...
 }
 irrecv.resume();
}