My friend sent me a strange text

Yesterday my friend sent me a really weird text. After that he wouldn't text me back or return my calls... Can you guys help me?

I've transcribed it to try to help...

[Blue Yellow][Blue][Orange Green Indigo][Green] [Green Green][Violet Violet Red][Violet Violet Blue][Violet Violet Indigo][Red][Violet][Blue] [Violet Violet Blue][Blue][Violet Violet][Green] [Blue Yellow][Blue][Violet Blue][Green Green Blue Yellow]!

I think that each section of brackets might be a word, but I'm not sure.

Hint 1:

My friend loves rainbows.

Hint 2:

He likes to create conversion puzzles. E.g. numbers to letters.

Hint 3:

My friend also likes math.

• Liked this one! QUick note though, you don't really need to add any hints as soon as you post, once the post goes a few hours, maybe a few days, then starting adding hints. Nice puzzle though :) – Beastly Gerbil Oct 28 '17 at 17:51
• To make things easier, I've abbreviated each of the colors: [BY][B][OGI][G] [GG][VVR][VVB][VVI][R][V][B] [VVB][B][VV][G] [BY][B][VB][GGBY]! – Mateen Ulhaq Oct 28 '17 at 23:13

The message is

HELD HOSTAGE, SEND HELP!

How the cipher works

Take the corresponding position number for each colour (ROYGBIV = 1234567) and add up all the numbers in each bracket. Finally, convert to a letter using A1Z26.

So:

Conversion tables:

ROYGBIV
1234567

A B C D E F G H I J  K  L  M  N  O  P  Q  R  S  T  U  V  W  X  Y  Z
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26
[BY] = [5+3] = [8] = H
[B] = [5] = E
[OGI] = [2+4+6] = [12] = L
[G] = [4] = D

[GG] = [4+4] = [8] = H
[VVR] = [7+7+1] = [15] = O
[VVB] = [7+7+5] = [19] = S
[VVI] = [7+7+6] = [20] = T
[R] = [1] = A
[V] = [7] = G
[B] = [5] = E

[VVB] = [7+7+5] = [19] = S
[B] = [5] = E
[VV] = [7+7] = [14] = N
[G] = [4] = D

[BY] = [5+3] = [8] = H
[B] = [5] = E
[VB] = [7+5] = [12] = L
[GGBY] = [4+4+5+3] = [16] = P
!

HELD HOSTAGE SEND HELP!