This isn't straightforward the way Flash does it. With flash a colortransform is
Y = C * colorMult + colorAdd;
This naturally requires 2 colors to be present. A unity mesh only supports 1 color attribute, and although its possible to hack it in with another attribute (eg. tangent), it is likely to use a LOT more memory like that. Also it'll be a nasty hack. I can show you how you'd do something like that using tangents, if you'd like to give it a go - its straightforward if you don't use some of the features of 2D Toolkit, mainly the generate tangents / normals one.
Alternatively, you can switch materials when you need to flash them. I posted a material to the Unity forum thread a while back, I'm just pasting it here should you wish to give it a go -
// unlit, vertex colour, alpha blended, additive overlay
// cull off
Shader "tk2d/BlendAdditiveVertexColor"
{
Properties
{
_MainTex ("Base (RGB) Trans (A)", 2D) = "white" {}
}
SubShader
{
Tags {"Queue"="Transparent" "IgnoreProjector"="True" "RenderType"="Transparent"}
LOD 100
ZWrite Off
Blend SrcAlpha OneMinusSrcAlpha
Cull Off
BindChannels
{
Bind "Vertex", vertex
Bind "TexCoord", texcoord
Bind "Color", color
}
Pass
{
Lighting Off
SetTexture [_MainTex] { combine texture + primary, texture * primary }
}
}
}
With this shader, setting the color will brighten the sprite (i.e. black = displays as normal, white = the sprite is white)